Rick Santorum has a challenge. He doesn’t need to catch Mitt Romney, but it would certainly help his cause to come in ahead of Newt Gingrich, who is vying for the not-Romney spot. Right now his poll numbers in South Carolina are troubling to his supporters. In the RealClearPolitics average he is tied in South Carolina with Rep. Ron Paul (R-Tex.) in fourth place with 14.3 percent. In Florida he’s in third, but more than 25 points back in the RCP average.

Meanwhile, Gingrich’s debate performance gave him a lift, and he has one more shot on Thursday to seal the deal with the voters. So what can Santorum do?

Most important, he should ignore the talk about Romney’s top marginal tax rate. Although the media are once again obsessed with Romney’s wealth, South Carolina voters in poll after poll approve of his business experience. Every moment talking about Romney’s income taxes is wasted. His argument is not: Romney is too rich. His argument is: Romney isn’t a consistent conservative.

Santorum would also do well to stop complaining about ads. No one ever got elected grousing about the other guy’s ads. Moreover, it forces Santorum back into his less attractive persona — grasping and angry. Calling the other guy a “liar,” as he has, brings back memories of Bob Dole. (“Stop lying about my record!”)

But there is plenty Santorum can do between now and Saturday. He can’t afford in the debate to concentrate purely on Romney. However, he might think about taking a page from Rep. Michele Bachmann’s playbook, specifically the page labeled “Newt/Romney.” Her argument was that both supported the individual mandate, both supported TARP and both have a habit of capitulating to Democrats (Romney to Sen. Ted Kennedy and the Massachusetts legislature and Gingrich to Bill Clinton.) It works because, while his record has some clunkers, Santorum is certainly to the right of both Gingrich and Romney.

In addition, he should pick up with where he left off on the Social Security argument. Santorum is right on the merits — we’d have to borrow trillions more from China to pay for individual accounts. It’s a perfect example of Gingrich’s fascination with unattainable ideas.

Santorum would also be smart to go after Gingrich, as he did yesterday for his erratic, reckless behavior: “Newt is bold, but he is all over the place. Attacking capitalism, supporting capitalism. Against global warming, for global warming. We need someone who is bold and consistent.” It’s also fair game to go after Gingrich’s behavior in office. He alone among the candidates has a track record of serious ethical violations and was kicked out of leadership by fellow Republicans.

It’s hard to fight a two-front war, but in essence that is what Santorum must do. He’s got to convince voters he can take it to Romney if it becomes a one-on-one face-off, and he’s got to knock Gingrich down to third place. If not, it will be hard to maintain the argument that he is the most viable alternative to the front-runnner.