This evening there will be another presidential debate, the first since the New Hampshire primary. Just as he was in the debates preceding New Hampshire, Mitt Romney is leading in the polls. However, in South Carolina there still is a large chunk of the electorate that is undecided or willing to change their pick. The debates tonight and Thursday are perhaps the last chances for his rivals to knock him off his stride.

Romney, of course, would prefer to let his opponents squabble among themselves. They might oblige, but he will certainly be their principle target. He is going to get grilled on the Bain-related questions. Won’t President Obama use this against you? How can you relate to working-class voters? (If the moderators or opponents decide to go after specific Bain deals, they are foolish; Romney has far more knowledge of these than they do and would leap at the chance to debunk the charges of the media or his opponents.) If he can continue to stand on the side of capitalism, casting his opponents as playing into voters’ fears like the president or being ignorant of the American economy (or both), he’ll be pleased.

More problematic for Romney is the assault that will come from the right, primarily from Rick Santorum. Once again, he’ll have to navigate around the Romneycare questions and make the case that he is more conservative and more consistent than he is portrayed. (Santorum is going to get pushback since he endorsed Romney in 2008. He better have a clever, non-defensive answer.)

More than for Romney, who likely can survive a bad debate performance, tonight is critical for Santorum in several respects. He will need to convince voters that the backing of evangelicals is significant (indeed game-changing) and makes him the only viable not-Romney contender. He is also going to have to convince them that he can take on Romney and then Obama rhetorically, zeroing in on their weaknesses and posing as the candidate most attractive to blue-collar workers.

Santorum must avoid three potholes: appearing too angry, sounding too verbose (especially on his economic plan) and getting dragged into tit-for-tats with candidates behind him in the polls. If he can do all that and show a presidential command of the facts and mature restraint, he can help himself greatly.

Newt Gingrich is most at risk here. He’s going to get dinged for his Bain attacks, grilled on his temperament and challenged perhaps to get out of the race to make way for Santorum. If he confirms voters’ worst fears by lashing out or trying to recast reality, he’ll suffer. He’d be wise to muster up some graciousness when he’s asked about his opponents (e.g., Romney’s back-to-back victories, Santorum’s evangelical backing). Even a good debate may not save him from his own missteps, but a bad one will certainly finish him off.

Another isolationist jag will hurt Rep. Ron Paul (R-Tex.), reminding voters why ultimately he is not electable. In the debate and in the remaining contests, he has a chance to play a constructive role, both in revealing the other candidates’ records and in cementing his own supporters’ attachment to the eventual nominee, who will lead the only major political party determined to limit the size of government. He serves another useful role: making all his opponents seem far more centrist than Obama likes to make them out to be. Neither Santorum nor Romney want to end entitlements, for example. They are not, as Obama charges, for a society in which it’s every man for himself. That’s Ron Paul, and the contrast between him and the eventual nominee should be instructive.

Then there is Texas Gov. Rick Perry, whose campaign is winding down in a somewhat pathetic fashion. He’s given his all to ingratiating himself with evangelicals (remember the prayer day before he entered the race?), and yet they snubbed him in Texas. He will return to Texas a diminished figure. Like Sarah Palin, he’ll have to decide if it is important for him to be taken seriously as a conservative leader. If so, he’s got some work to do.He’s in a tricky place now, realizing that one of the men on the stage will be the nominee and potentially the president. Perhaps he’ll set aside his attacks (which haven’t worked anyway) and try to finish this week on a high note. It will help him as he sets out to repair his political reputation and determine his future, if any, in the Republican Party beyond Texas.