David Goldman, AP
Republican presidential candidates former Pennsylvania Sen. Rick Santorum, former Massachusetts Gov. Mitt Romney and former House Speaker Newt Gingrich shake hands before the start of the Republican presidential candidate debate at the North Charleston Coliseum in Charleston, S.C., Thursday, Jan. 19, 2012. (AP Photo/David Goldman)

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NORTH CHARLESTON, S.C. — Mitt Romney entered the final full day of campaigning in South Carolina's GOP primary contest Friday scrambling to fend off challenges from more conservative rivals Newt Gingrich and Rick Santorum and insisting he's the one that Republicans can trust to "post up well" against President Barack Obama in the fall.

With South Carolina's critical primary only a day away, Gingrich was gaining on Romney, and South Carolina's Sen. Jim DeMint declared it a "two-man race." Santorum insisted he's still part of the equation and that he's finally drawing enough campaign contributions to compete aggressively in next-up Florida and beyond.

Rick Perry's departure from the race, a raucous Charleston debate and fresh reminders of Gingrich's tumultuous personal life promised to make the dash to Saturday's voting frenetic and the intra-party attacks increasingly sharp.

But Republican Party Chairman Reince Preibus, in a morning appearance on CNN, insisted the tone wasn't all that negative and said "a little bit of drama" was good for the GOP as it sorts out the strongest challenger to Obama.

Romney, appearing on Fox News Channel, called Gingrich "a feisty competitor" but argued the former House speaker was not the best man to put up against Obama. Santorum, who turned up on C-SPAN, said the GOP presidential race "has just transformed itself in the last 24 hours" and that's he's still very much part of the mix.

At Thursday night's debate, he offered himself as a more reliable conservative than either Romney or Gingrich.

"I've been fighting for health reform, private sector, bottom-up ... for 20 years, while these two guys were playing footsies with the left," Santorum said.

Romney, whose lead has shrunk in the race's closing days, opened the day by waving the endorsement of Virginia Gov. Bob McDonnell and planned stops along the coast, in the state's midlands and conservative north. Gingrich, buoyed by Perry's endorsement, concentrated on the south, especially the heavily pro-military Charleston area.

But the former House speaker canceled a planned Friday morning appearance at the Southern Republican Leadership Conference due to poor attendance. Gingrich spokesman Nathan Naidu said the decision had been reached in conjunction with conference officials. There were only about two dozen people in the hall at the College of Charleston.

Santorum, the former Pennsylvania senator, was rallying supporters in four stops statewide, including the conservative upstate, the home of his evangelical base. The libertarian-leaning Ron Paul, whose support has slipped with his light campaign effort here, hoped to whip up his supporters with a six-city fly-around.

Saturday's victor "is likely to be the next president of the United States," DeMint, a tea party leader, predicted on "CBS This Morning." He hasn't endorsed a candidate in the race.

The GOP race spun wildly Thursday, beginning with news that Santorum had edged Romney in Iowa, a reversal of the first nominating contest more than two weeks past.

Perry, having struggled in vain to build support in his native South, quit and endorsed Gingrich. Gingrich, meanwhile, faced stunning new allegations from an ex-wife that he had sought an open marriage before their divorce. An aggressive debate punctuated the day.

Santorum played aggressor during the faceoff, trying to inject himself into what seemed increasingly like a Romney-Gingrich race after Perry's endorsement of his onetime rival.

"Newt's not perfect, but who among us is," Perry said in backing Gingrich. "The fact is, there is forgiveness for those who seek God and I believe in the power of redemption, for it is a central tenet of my own Christian faith."

Gingrich angrily denounced the news media for putting his ex-wife front and center in the final days of the race and spreading her accusations. "Let me be clear, the story is false," he said when asked at the opening of the debate about her interview.

Santorum, Romney and Paul steered clear of the controversy.

"Let's get onto the real issues, that's all I've got to say," said Romney, although he pointed out that he and his wife, Ann, have been married for 42 years.

Gingrich and Santorum challenged Romney over his opposition to abortion, a well-documented shift but a potent one in evangelical-heavy South Carolina.

Recent polls, coupled with Perry's endorsement, suggested Gingrich was the candidate with the momentum and Romney the one struggling to validate his standing as front-runner.

Gingrich released his income tax records during the course of the debate, paving the way to discussing Romney's. The wealthy former venture capitalist has said he will release them in April, prompting Gingrich to suggest that would be too late for voters to decide if they presented evidence Obama could exploit.

"If there's anything that's in there that's going to help us lose the election, we should know before the election. If there's not, why not release it?" Gingrich said. His effective tax rate, roughly 31.6 percent of his adjusted income, was about double what Romney told reporters earlier this week he had paid.

Romney, asked about the issue Friday on Fox, said he didn't want to give Obama and the Democrats a "nice little present of having multiple releases." He said that past GOP nominees John McCain in 2008 and George W. Bush before him released their taxes at tax deadline time, and said he'd do likewise. He didn't say how many years of returns he would release.

Gingrich grappled with problems of a different, possibly even more crippling sort in a state where more than half the Republican electorate is evangelical.

Marianne Gingrich told ABC's "Nightline" that her ex-husband had wanted an "open marriage" so he could have both a wife and a mistress. She said Gingrich conducted an affair with Callista Bistek, now his wife, "in my bedroom in our apartment in Washington" while she was elsewhere.

"He was asking to have an open marriage and I refused," she said.

Santorum, asked about the issue Friday on C-SPAN, said it would be up to voters to decide "whether these are issues of character" that matter in the race. But he said that when such actions occur when someone is serving in public office, "that has an additional level of relevance."