Tonight’s debate is Rick Santorum’s last real shot to make his pitch as the viable not-Romney candidate. For goodness’ sake. If he can’t elbow Newt Gingrich out of the way with Gingrich’s ex-wife Marianne metaphorically hovering in the wings, when is Santorum going to put him away?

As Ross Douthat writes:

Santorum is less compromised on the crucial issues of this campaign cycle, less detested by conservative insiders, and less weighed down by the baggage that drags behind Gingrich like Jacob Marley’s moneyboxes.

If it hadn’t been for Gingrich’s persistence, Santorum might have had a brief window to frame the primary campaign the way it was expected to be framed: As a referendum on Romney’s liberal past, rather than on the weakness of the more conservative alternatives. And this, in turn, might have forced Romney into a much longer campaign than the smooth ride he’s enjoying

To do that Santorum has to create some big moments and exploit Gingrich’s obvious vulnerabilities. Let’s face it: Santorum’s pitch is that Gingrich is erratic, not all that conservative and lacking character. He’s going to have to find ways to say that or he’ll be looking at third (maybe a distant third) place on Saturday. That means raising Gingrich’s rollercoaster speakership, his reckless personal life, his propensity to snuggle up to liberals and his dramatic policy shifts (for the individual mandate before he was against it).

It certainly would help if Santorum could jettison the whining about Mitt Romney’s super PAC and stop dwelling on minor issues. (Felon voting rights? C’mon.) As Tim Pawlenty found out, a candidate can look very solid on paper but lack the force of personality and sense of command voters look for in a president. Santorum can certainly go on beyond South Carolina, but his task becomes infinitely harder with a weak showing on Saturday.

As for Gingrich, his task is to survive the onslaught with calm and without defensiveness. He’ll need to be careful about carping at his ex-wife or blaming her reappearance on a media conspiracy. Likewise, on the attacks on his speakership rather than trashing the messengers (Susan Molinari and Jim Talent) he’d be wise to ‘fess up: He was a flawed leader and he learned from that experience. He might want to reel in his claims to have created all those jobs as well. That, along with his claim to have helped end communism, tends to make him appear a tad egocentric. Just a tad.

His last string of Bain attacks are also problematic for him. He’s on shaky ground on his facts. And with the emergence of an interview by Paul Levy recounting Gingrich’s speech to a private-equity group, Gingrich is vulnerable to the charge of hypocrisy and opportunism. He will try to wiggle out from his rhetoric or recast it. If Romney is on his game he’ll push him a bit and force him to either retreat from his accusations or embrace his cartoon version of capitalism, which is both untrue and politically obnoxious to conservatives. If he can leave the stage without too many YouTube-able moments (the bad kind) he’ll be fortunate.

For Romney, the temptation is to go over Gingrich’s head and direct his comments toward the general election and President Obama. But he has, in the Bain episode, a unique opportunity to puncture Gingrich’s pretense of financial sophistication and to remind voters that, as in Gingrich’s attacks on Rep. Paul Ryan (R-Wis.), this is a man who will say or do anything to promote himself. As for all the issues about his taxes, Romney should be succinct and promise to release his returns by a date certain. He might take the time to explain why capital gains are taxed at a lower rate and why actual conservatives should work toward lowering the cost of capital. Mostly, Romney would do well to be the calm in the storm. Voters should know he is not going to embarrass them or run an erratic general-election campaign.

Finally, Rep. Ron Paul (R-Tex.) is with each debate and election becoming more and more a fringe candidate. He can play a role in defending conservative economic principles (sound money, free markets). But he risks losing whatever leverage he might seek (a convention speech slot, for example) by reverting to his loony national security rants.

Each debate and primary race is considered critical — until the next one comes along. But South Carolina can surely help shape the races that follow. At the very least we’re in all likelihood going to have at least one candidate drop out. The winner will have bragging rights for 10 days. It may not be everything, but it is something.