The last debate before the crucial South Carolina primary started off with a bang when CNN host John King asked Newt Gingrich about his second wife’s charge that he asked for an “open marriage.” The former speaker responded with a tirade against the media that earned wild applause from the audience in Charleston and may well have been the most significant sound bite from the evening. But the rest of the night didn’t go quite as well as for Gingrich, who entered the evening leading in some of the latest polls in the state.

The reason for that was this turned out to be Rick Santorum’s strongest performance in any of the debates. The former Pennsylvania senator scored points all night at the expense of both Gingrich and Mitt Romney, who spent much of the night on the defensive. That’s problematic for Romney, who might be able to salt away the nomination with a win on Saturday night. But the question for Republicans is whether Santorum’s pounding of the two men ahead of him in the polls will take away enough votes from Gingrich to let Romney squeak out a win in the state. Even more importantly, they will be left wondering whether Gingrich will be able to get away with dismissing his ex-wife’s comments as “trash” if a win in South Carolina enables him to effectively challenge Romney for the nomination.

Gingrich’s response to questions about his personal past was a classic case of misdirection in which he castigated the media for reporting the accusations and then, almost in passing, denied that Marianne Gingrich was telling the truth. He claimed his friends could disprove what she said, but because the conversation she has mentioned was private, it’s difficult to see how that could be true. But because Republican resentment of the media always runs white-hot and it can, in truth, be argued the timing of the broadcast of the interview was prejudicial, it’s unlikely it will cost him the primary. Though the indignant manner in which Gingrich sought to deflect the question worked well in the hall, it could come back to haunt the GOP if the former speaker prevails in the Republican contest. If Gingrich thinks he can put a revelation like this in the past by merely yelling at a questioner, he’s mistaken.

That exchange may well dominate coverage of the debate, but the rest of the evening could be said to belong to Santorum. Even though he was officially informed today that he won Iowa after all, Santorum knows his campaign is on life support if he finishes a distant third or even fourth in South Carolina. So he came out swinging at both Romney and Gingrich, flaying them on health care and immigration. He launched an especially devastating attack on Gingrich’s leadership qualities and character in which he lampooned his “grandiosity” and unreliability, not to mention his chutzpah in asking Santorum to withdraw after being bested by him in the first two states to hold primaries.

Characteristically, Gingrich responded by validating the charge by taking credit for the Reagan presidency, the defeat of the Soviet Union, the 1994 GOP congressional victory and anything else he could think of.  The exchange revealed Gingrich’s boundless vanity and utter lack of self-awareness and even allowed Romney one of his few good moments of the evening in which he ridiculed Gingrich’s willingness to take credit for things for which he had little responsibility.

As for Romney, though he had his moments, especially when fending off ill-advised Gingrich attacks from the left on his business career, it was another off night. He took a beating on the question of releasing his tax returns and even was heckled by the crowd at one point. He’s right that it’s a marginal issue that plays to Democratic prejudices, but he’s foolish to not just release the returns and get it over with, especially since no one suspects he has anything to hide.

The evening was also distinctive from previous debates as Ron Paul went the entire two hours without a rant about the Federal Reserve or rationalizing America’s enemies abroad. He even scored a rare point at Santorum’s expense when he pointed out the folly of opposing global trade.

If the evening was to be judged on the basis of whether Romney got the initiative back from Gingrich, it had to be judged a failure for the former Massachusetts governor. He has to hope Santorum’s good showing results in a loss of momentum for Gingrich. Though Gingrich ought to see the debate as a very mixed bag for him, he is clearly counting on the backlash against the media’s interest in his personal life to enable him to continue to avoid answering the tough questions about his behavior while he makes more “grandiose” statements about beating Obama in future debates.

As for Santorum, though he did well, he also sounded at times as if he knew the end was near, especially when he expressed gratitude for making the “final four” of the contest. The valedictory note may have been premature, but unless Santorum comes from out of nowhere on Saturday, there’s a real possibility the field may soon be winnowed down to a final three.