It’s utterly appropriate in a campaign that of late has been saturated with religion that tonight’s potentially decisive debate is being held on Ash Wednesday. And the Ash Wednesday debate is absolutely critical to Rick Santorum.

Mitt Romney’s campaign has been exceptionally clever in the last week, and Santorum has played into Romney’s hands. It’s striking that conservative Web sites sympathetic to Romney have dumped out all sorts of old videos of Santorum waxing very right-wing on matters such as contraception and the family — and even a sermon he delivered on Satan. Having spent two years covering the Vatican (I even wrote news stories on Satan), Santorum’s talk about the Evil One didn’t surprise me. But it does sound very strange in the context of a presidential campaign.

Never one to run from a fight, Santorum has continued to speak out on these themes, reinforcing his standing as a social and religious conservative so staunch that he would prefer to lose an election than give up on his core beliefs. This has allowed Romney to perform some jujitsu. His fingerprints are not on any of the reports or criticisms of Santorum’s eagerness to run toward the religious right. This has all been handled by surrogates. But Romney has subtly suggested that Santorum is too conservative to beat Obama with such oblique comments as his recent declaration that Santorum has not been “as carefully viewed by the American public” as other candidates. It’s Romney’s invitation to Republican primary voters to take a look at all those videos.

My sense is that Santorum’s social issue extravaganza has put him in danger of losing the Michigan primary. There are plenty of quite conservative Republican women who may now view Santorum as a step too far. They could add to Romney ballots already in the bank from early voting.

Santorum has more than made his point to religious conservatives that he is one of them. Tonight, he needs the discipline to appeal to more moderate conservatives who may appreciate his blue-collar roots and find him more authentic than Romney. Yes, Santorum needs to show he can move to the center at least a little bit and be a reassuring presence. And he needs to find a way to put Romney on the defensive quickly.

Many Republicans doubt that Santorum can win the fall election, and for good reason. How Santorum performs against Romney in this debate will be a good test of his ability to transition from the leader of a moral crusade into a presidential campaigner. As for Romney, he has to hope that Santorum is asked one question after another about his religious and moral views. Santorum may not be able to resist the temptation (which he sees as an obligation) to defend his faith, and his vision of morality.