Which is why tonight’s debate, the final televised encounter between the candidates before next Tuesday (and before Super Tuesday, for that matter), looms so large. Debates have played an outsize role in shaping mass opinion in this campaign, and in helping Romney fend off his various chief rivals of the moment.

At various times, different candidates have shone while playing supporting roles — standing off on the side of the stage, receiving little attention from Romney and their fellow candidates, but managing to catch the audience’s interest with a compelling answer or two. This is how Herman Cain and Newt Gingrich both came to life as candidates, and it may help explain why Santorum was suddenly able to gain traction these past few weeks. But when they’ve had Romney’s (and the debate moderator’s/media panel’s) full attention, it’s been a different story, with one Romney foe after another wilting under the spotlight.

Part of the promise of Santorum’s candidacy has been that he’ll avoid the Debate Curse – that he’s actually a competent communicator who can handle the heat from Romney and everyone else that comes with being a front-runner and that he might actually walk away from the debate experience enhanced, not diminished.

To put this in some perspective, here’s the history of debate futility by Romney rivals that Santorum will try to defy tonight:

Romney vs. Tim Pawlenty

Pawlenty was not surging in the polls as Santorum is now, but he was widely seen as Romney’s most serious threat last spring and summer, meaning that he was in the spotlight when the candidates debated.

June 13: After telegraphing his intent to hit Romney over his healthcare plan (“ObamneyCare,” in Pawlenty’s telling), the former Minnesota governor is offered an opportunity to confront his opponent during the debate … and completely chickens out.

Aug. 11: Pawlenty still had no traction when this debate began, and with the Ames straw poll 48 hours away, he largely left Romney alone and focused his attacks on a more immediate threat, Michele Bachmann. It didn’t go very well, and by the end of the weekend Pawlenty was out of the race and a Romney threat had been permanently extinguished.

Romney vs. Rick Perry

Pawlenty’s failure to catch fire helped encourage Perry to enter the race, and the Texan immediately shot to the top of the polls. He seemed to be the magical candidate the race had been missing, someone who had natural appeal to the Romney-base but who was still credible enough to make the establishment comfortable.

Sept. 7: Perry’s first debate as a candidate isn’t a disaster, but his uneven performance – and especially his effort to defend his climate change skepticism by arguing that “Galileo got outvoted for a spell” — provides hints of trouble to come.

Sept. 12: More signs of trouble, with Perry struggling to offer a defense that resonates with conservatives of the HPV vaccination bill he signed in Texas. This results in an extended back-and-forth with Michele Bachmann, who wins loud approval from the crowd as she frames the vaccine as government overreach. And when she notes that Perry received campaign contributions from the vaccine-maker, he awkwardly replies that “If you’re saying I can be bought for $5,000, I’m offended.”

“I’m offended for all the little girls who didn’t have a choice,” Bachmann replies. The impression that Perry may not be very quick on his feet begins to take hold.

Sept. 22: Perry officially blows it, failing in painful fashion to recite an extremely basic attack on Romney’s conservative credentials:

I think Americans just don’t know sometimes which Mitt Romney they’re dealing with. Is it the Mitt Romney that was on the side of against the Second Amendment before he was for the Second Amendment? Was it — was before he was before the social programs, from the standpoint of he was for standing up for Roe v. Wade before he was against Roe v. Wade? He was for Race to the Top, he’s for Obamacare, and now he’s against it. I mean, we’ll wait until tomorrow and — and — and see which Mitt Romney we’re really talking to tonight.

Afterward, the conservative pile-on begins, and the Romney campaign sighs with relief as Perry’s poll numbers crash.

Romney vs. Herman Cain

As Perry flames out, Cain starts to catch the right’s eye from his supporting role. By the middle of October, he’s left Perry in the dust and is threatening to do the same to Romney.

Oct. 18: But now that he’s in the spotlight, Cain finds himself facing aggressive questions from Romney and the other candidates, and from the debate moderator. Republicans quickly discover that he’s incapable of providing a simple, coherent defense of his 9-9-9 tax plan – and that he has literally nothing to say about anything else:

Romney vs. Perry (again)

A sexual harassment scandal accelerates Cain’s demise, once again leaving Romney as the clear front-runner – and stirring talk that Perry just might be able to earn a second look from Republicans, especially if he can step up his debate game …

Nov. 9: … but instead, this happens:

Romney vs. Newt Gingrich

With Cain and Perry going nowhere, the former speaker, who excels like no one else at making noise from the edges of the debate stage, moves into contention. By early December, he’s opened big leads over Romney nationally and in most of the early states, leading him to boast that “it’s very hard not to look at the recent polls and think that the odds are very high I’m going to be the nominee.”

Dec. 10: Romney comes after his newest chief rival hard in the early going, but Gingrich holds his ground fairly well. And then Perry, of all people, somehow gets under Romney’s skin, leading Romney to offer a $10,000 bet that serves as the debate’s headline moment. This may be the only debate in which Romney’s chief foe walked away in better shape than he entered.

Dec. 15: Again Romney shows up ready, and this time he avoids any gaffes. Meanwhile, Gingrich seems oddly passive and unwilling to engage with Romney’s attacks, much less lob any of his own. He seems to believe that he’ll be rewarded by GOP voters if he’s seen as running an exclusively positive campaign, and when the debate is over, he even tries to take back criticisms of Romney he’d made days earlier. This is the last debate Iowans see before their Jan. 3 caucuses – but in the weeks in between, they do see countless anti-Gingrich ads on their television screens.

Mitt vs. Gingrich (again)

Newt fades badly in Iowa and New Hampshire and his candidacy seems near its end. But Santorum fails to parlay his Iowa victory* into a credible showing in New Hampshire, which allows Gingrich to get back in the game in South Carolina – where he steals the show at two debates as Romney, still hoping to post a clean, campaign-ending victory, refrains from playing attack dog. That sets the stage for Florida, which suddenly becomes a do-or-die test for Romney.

Jan. 23: Faced with a clear and immediate threat, Romney punches away at Gingrich intently, but the former speaker is dull and unfocused in response. The contrast is jarring, and devastating to Gingrich, who blames debate sponsor NBC’s efforts to keep the live audience quiet.

Jan. 26: Gingrich tries a trick that has worked brilliantly for him before – picking a fight with the moderator – but this time it backfires miserably. Once again, the night belongs to Romney, who shreds Gingrich with the focus and intensity of a seasoned prosecutor. And once again, Gingrich seems incapable of mustering any comebacks. When the debate is over, it’s clear that Romney is on his way to victory in the Sunshine State

* * *

And now it’s Santorum’s turn. Because of the circumstances of his Iowa victory, he’s never had Romney’s attention at a debate like he’ll have it tonight. At times in previous debates, he’s mixed it up with Romney and shown the potential to be a far more dangerous challenger than Perry, Gingrich or Cain. But those were isolated exchanges. He’s never been in the spotlight like this before.

This is what made Santorum’s initial pledge not to make social issues the focus of his Michigan campaign a serious problem for Romney. Conservatives who vote on cultural issues already didn’t trust Romney and already knew that Santorum was one of their own. So by playing up his economic message (and his own middle-class identity), Santorum would have a chance to make Romney’s problem with blue-collar and middle-class voters worse while expanding his own base and showing party leaders that he’s more than a niche candidate and that they needn’t fear nominating him.

Instead, just about every headline that Santorum has generated since then has been about culture war politics, a trend that has accelerated in the past few days as he’s blasted Barack Obama’s “phony theology,” railed against mandatory coverage of prenatal care, and even made a Hitler reference. There’s also the stir that one of his aides caused on Monday by warning on national television about Obama’s “radical Islamic policies.” (She meant to say “environmental policies,” the aide claimed.) And these are just the highlights.

I previously theorized that Santorum might be doing this because he’s worried about anti-Romney evangelical voters — particularly in the South — voting for Newt Gingrich, whose candidacy was just tossed another lifeline by Sheldon Adelson, and splitting the conservative base. Or maybe he’s just doing it because he’s Rick Santorum and he can’t help himself. Whatever the explanation, his refusal to rein himself in after his “phony theology” remarks first attracted notice is drowning out whatever he has to say about economic issues and reinforcing his image as a culture warrior politician.

This poses two problems for him. One is that the negative tone of the headlines he’s generating — at one point Monday, the top of the Drudge Report was monopolized by links about Santorum controversies — could be alarming to Republican voters in Michigan (and Arizona, for that matter) who aren’t part of the hardcore base. That is to say, conservative voters who have their doubts about Romney and are receptive to an alternative might now conclude that Santorum is no more credible than Gingrich, Perry or Cain. There are now clear signs that his lead over Romney in Michigan is vanishing. Romney’s heavy television presence in the state probably explains most of this, but Santorum’s antics threaten to exacerbate the trend.

The bigger problem has to do with the GOP’s opinion-shaping elites — elected officials, party leaders, activists, commentators, media personalities whose views tend to trickle down to the rank-and-file. An unusually high number of elites have remained on the sidelines so far. Since grabbing the spotlight two weeks ago, Santorum has essentially been auditioning for them. If he can show them he’s for real, their support would radically enhance his prospects of winning the nomination, amplifying his own message, giving him invaluable cover against Romney attacks, and funneling serious money into his campaign (and a super PAC). But so far, he’s won very few new converts; Mike DeWine, the former Ohio senator and current attorney general, is the highest-profile endorsement Santorum has snagged.

So for now, at least, Santorum is virtually alone as he confronts Romney’s money and attacks — and now a days-long media firestorm over his own comments. It’s not a myth that there was, and is, an opening to Romney’s right. But Santorum may learn that even in the GOP of 2012, there’s such a thing as too much red meat.

But the fact remains that Romney has trailed in every Michigan poll taken recently, with the newest numbers from Public Policy Polling putting him 4 points behind. And his edge over Santorum in the most current Arizona survey is just 3 points, even though Santorum hasn’t put up much of a fight in the state. With Santorum now opening a double-digit lead nationally and with a pivotal debate set for Wednesday night, it’s absolutely possible that Romney will fall short in his native state, Michigan. And it’s not impossible, though far less likely, that he’ll lose in Arizona as well.

This is why there’s suddenly loud talk about a new candidate jumping into the GOP race. If Romney melts down, Santorum looms as the next most likely victor — and his white hot culture war rhetoric these past few days is a perfect demonstration of why most November-minded Republicans believe his nomination would be a disaster. And after Santorum comes Newt Gingrich, whom those same Republicans tend to regard as poison, and then Ron Paul, who’s a nonstarter. As an unnamed Republican senator told ABC News late last week, “If Romney cannot win Michigan, we need a new candidate.”

But while the GOP’s need for a savior would be obvious, don’t count on one emerging in the event of a Romney meltdown next Tuesday. There are four major reasons why:

1. There just isn’t one. It’s been obvious since early last year that there was room for another candidate in the race, one capable of exciting the GOP’s Tea Party base while earning and maintaining the approval of the party’s opinion-shaping elites. This is easier said than done, as Rick Perry painfully demonstrated. The three most commonly mentioned white knights are Jeb Bush, Mitch Daniels and Chris Christie, but each has ideological baggage of his own; whether any is actually what the base is looking for is an open question. And after them, who is there? At this very, very late point, any new candidate would need to demonstrate wide support immediately. There just aren’t many — or any — Republicans who could do this.

2. It’s gotten too complicated. But let’s say that Jeb or Chris or Mitch (or whoever) wanted to get in anyway. We’ve long since passed the point at which a new candidate would be able to qualify for enough state ballots to have a shot at securing a delegate majority during the primary season; so the only hope for a white knight now would be to get on the ballots he (or she) could, hope to secure enough delegates to keep any of the other candidate from gaining a majority, then take a shot at a deadlocked convention. In other words, even if the new candidate caught fire and won some late primaries, securing the nomination would still be a difficult, maybe even impossible, task. Bush, Christie and Daniels will all still be viable in 2016 (if the GOP nomination is open then) and already decided not to run in 2012, back when the path to the nomination was much clearer than it is now. They’d each have an enormous amount to lose by making a last-minute stab at this year’s prize.

3. What about the guy who’s already running? The white knight talk is based on an assumption that Romney would be mortally wounded by a loss in one or both states next week, and maybe he would be. But what would happen to Santorum if he won one or both? He’s already climbed into the mid- to high-30s in national GOP polling and demonstrated an ability to connect with the conservative base. Slaying Romney would probably make him a rock star on the right — and would make it easy for him to stoke resistance to any new candidate, whom he could portray as the tool of a desperate, soulless establishment.

4. The GOP nomination isn’t quite what it looked like before. President Obama’s political standing has markedly improved in the past few months, likely a combination of good economic news and negative headlines about the GOP race. His gains are tenuous and could easily be wiped out if the recovery stalls in the months ahead, but if they continue, the GOP nomination this year could end up being as valuable as it was in 1996, when Bob Dole never stood a chance against Bill Clinton. This is all the more reason for ambitious Republicans who are on the sidelines now to stay there and wait for ’16.

Again, the talk of a new candidate will die down if Romney can right his ship and fend off Santorum in Michigan and Arizona. But even if he doesn’t, here’s betting it still won’t amount to anything.

The assumption is that Romney’s Mormonism plays a significant role in this. The bulk of southern evangelicals are Baptists and Pentecostals, who tend to be extra-resistant to the idea that the Mormon and Christian faiths are at all compatible and who may be extra-reluctant to vote for a candidate they consider so fundamentally different from them. As Robert Jeffress, the Texas Baptist leader who has called Mormonism “a theological cult,” put it a few months ago, “to those of us who are evangelicals, when all other things are equal, we prefer competent Christians to competent non-Christians who may be good, moral people like Mitt Romney.”

So with upcoming primaries in Georgia, Oklahoma, Tennessee, Alabama, Mississippi and Louisiana, there’s a real opportunity for Rick Santorum to harness this Romney resistance and do some real damage. But because of his northern Catholic background, Santorum isn’t a natural fit for southern evangelicals either.

So far, Santorum’s success has been limited to the Midwest, where he’s won contests in Iowa, Missouri, Minnesota and Colorado. But in South Carolina, he netted just 17 percent, good for a very distant third place, and in Florida he finished with 13 percent. In both states, evangelicals were more interested in Newt Gingrich, a Catholic convert who at least has southern political roots, than Santorum. And now that he (or, more precisely, the super PAC that’s aligned with him) is armed with another $10 million of Sheldon Adelson’s money, Gingrich poses a threat to Santorum’s southern hopes. If evangelicals really are more comfortable with the former House speaker and see him as viable, it could split the conservative vote throughout the South and open the door for Romney to win states with well under 50 percent of the vote.

Against this backdrop, the stir that Santorum caused over the weekend, when he claimed that President Obama is advancing “some phony theology,” takes on new significance. Santorum later claimed that he was actually talking about environmentalism, and not religion, but whatever his intent, the controversy could have the effect of strengthening his bond with evangelicals, particularly those in the South. Santorum also suggested that Obama practices a different form of Christianity and, in a separate weekend appearance, talked up the virtues of home-schooling. In a way, this was Santorum delivering a simple message to conservative evangelicals: I’m one of you. And it’s the kind of rhetoric that Romney, who has been trying to keep one eye on the general election, isn’t nearly as comfortable employing.

Gingrich’s success in South Carolina and the Florida panhandle suggests that Catholicism by itself isn’t enough to keep southern evangelicals from viewing a candidate as a “true” conservative. Santorum has the added obstacle of his northern roots, but there’s reason to believe he could run well — very well — in the Deep South in March. He’ll need to retain his viability (meaning no meltdown in Michigan) between now and then, but if he does, the red meat he tossed out this weekend should be very helpful.

Here’s a look at the Southern states that will vote in March. The first number is the share of GOP primary voters who identified themselves as evangelicals in 2008; the second number is the share of the statewide vote Romney received in ’08. Note that Virginia, which votes on March 6 and where 46 percent of the ’08 primary voters were evangelicals, isn’t listed, since Santorum failed to make the ballot there.

Oklahoma, March 6: 72%/25%

Tennessee, March 6: 73%/24%

Georgia, March 6: 62%/30%

Alabama, March 13: 77%/18%

Mississippi, March 13: 69%/***

Louisiana, March 24: 57%/***

***Romney dropped out before state voted

Since the Florida primary, Adelson has been sending signals that he understands Gingrich is very unlikely to win the Republican nomination, that he is perfectly comfortable with Romney being the nominee, and that he doesn’t want to hurt Romney’s long-term prospects of beating President Obama.

There are also signs that Adelson, whose political involvement stems mainly from his hawkish views on Israel and the Middle East, isn’t enthusiastic about the man who has supplanted Gingrich as Romney’s chief GOP foe, Rick Santorum. As the Wall Street Journal reported earlier this week:

Mr. Adelson doesn’t oppose Mr. Santorum, but he doesn’t share the former Pennsylvania senator’s socially conservative positions, including his strong antiabortion views, associates said. Mr. Santorum was one of only two Republicans who didn’t meet with Mr. Adelson in October around the time of a candidates’ debate in Las Vegas, according to a person familiar with the matter.

Viewed in this light, Adelson’s new $10 million gift could be seen as an indirect contribution to Romney. After all, Gingrich seems to run best among religious conservatives in South, where several states will vote in early and mid-March. Santorum, who has already demonstrated strength in the Midwest, badly needs to fold those southern voters into his campaign if he’s going to have a real shot at knocking off Romney. A revived, or partially revived, Gingrich could severely complicate this task.

By the same token, a revived Gingrich probably wouldn’t be much of a threat to Romney, who has had little trouble beating back the former speaker during his two previous surges. Gingrich, with all of his personal and ethical baggage and with all his influential enemies within the Republican Party, is a much easier target than Santorum for the Romney campaign.

The ideal scenario for Romney would be for Gingrich and Santorum to be equally viable in the upcoming contests, splitting up the conservative vote and allowing Romney an easy path to victory. But if he has to have one main conservative foe, he’ll be much better off if it’s Gingrich. To the extent Adelson’s $10 million makes either of these scenarios more likely, it’s a huge boost for Romney.

What may be most interesting here is the apparent ideological gulf between Gingrich’s actual voters and his chief financial backer. Gingrich has been pitching himself as the candidate for conservatives who want a true believer and who see Romney as the embodiment of a Republican establishment that doesn’t really share their values. But it turns out that the guy keeping the Gingrich campaign alive is precisely the kind of Republican they suspect Romney is.

By any reasonable standard, the federal government’s auto industry bailout has been a smashing success, but to admit this Romney would be admitting that he was wrong in a big way. And not just on any issue. After all, Romney grew up in Michigan as the son of the president of American Motors, and he presents himself to voters as a turnaround specialist whose unique insight into the world of business and industry will translate into a booming economy if he’s elected president. But now it looks like Barack Obama (and George W. Bush, for that matter) knew better than him.

So as he campaigns in Michigan now, Romney is playing dumb, pretending that the auto industry’s comeback actually validates what he said in ’08 and that Obama — whose administration nudged GM and Chrysler into structured bankruptcy — was actually following the course that he prescribed. The only difference in their approaches, as Romney tells it, is that he would have forced the auto companies to rely on private financing to get through the bankruptcy process, and not the federal government. Which, as Jonathan Cohn points out, would probably have been catastrophic:

In late 2008 and early 2009, when Chrysler and GM ran out of money, private financing was not available. Remember, this was not long after Lehman had collapsed and the entire financial industry was on the brink of collapse. Had the car companies attempted to reorganize through bankruptcy on their own, it’s quite likely they could not have found the money to continue operations. That would have meant liquidation and mass layoffs. Chrysler and GM would be gone, sending an economic shockwave through the entire Midwest and possibly the whole country. Every company connected to the domestic auto industry would have suffered, which is why Ford, although healthy enough to survive without federal assistance in 2009, supported the Obama rescue.

Needless to say, the news of GM’s profits couldn’t come at a worse time for Romney, who is now running behind Rick Santorum in Michigan. But even if he comes back and wins the state on Feb. 28 and grabs the Republican nomination, his bailout headaches won’t go away. The revival of the auto industry is one of the signature triumphs of Obama’s first term, something he and his campaign will spend the summer and fall reminding Americans of. And it will make for the perfect rebuttal for Obama every time Romney attacks him for his (supposedly) anti-business, jobs-killing agenda.

In a way, Romney’s predicament calls to mind the position Republicans were in when Bill Clinton ran for reelection in 1996. By that point in Clinton’s term, the economy was strong and deficits were shrinking. This was problem enough for the opposition party, but it was made worse by the fact that literally every Republican in the House and Senate had voted against Clinton’s 1993 economic program — with many loudly warning that it would cost millions of jobs, run up the deficit and plunge the country into a painful recession.

At the time, that posturing had been beneficial to the GOP, which won control of the House and Senate in 1994 in part by accusing Clinton of enacting the largest tax increase in history. But by ’96, with Americans feeling good about the direction of the country and none of the dire ’93 predictions coming to pass, Clinton was able to rub it in Republicans’ faces, and there was really nothing for them to say back.

The same is probably true for Romney now. It was easy to oppose the bailouts a few years ago, but there just isn’t much he can say now.