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Matt Rourke, Associated Press
A supporter of Republican presidential candidate, former House Speaker Newt Gingrich, carries a sign at the Southern Republican Leadership Conference, Friday, Jan. 20, 2012, in Charleston, S.C.

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 a day away and Gingrich gaining on Romney in recent days, 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, even if he finishes poorly in South Carolina.

Rick Perry's departure from the race, a raucous Charleston debate on Thursday 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.

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

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. His surrogates used a morning conference call with reporters to run down Gingrich for overseeing rampant spending on lawmakers' special projects when he was House speaker. Rep. Jeff Flake, R-Ariz., called Gingrich "the granddaddy of earmarks." Rep. John Campbell, R-Calif., said Gingrich was "the guy who began the process which led to the debts and deficits that we have."

Santorum, who turned up on C-SPAN, said the GOP presidential race "has just transformed itself in the last 24 hours" and that he's still very much part of the mix.

At an appearance in Lexington, he offered himself as a just-right "Goldilocks" candidate, positioned between Gingrich and Romney.

"One candidate is too radioactive, a little too hot," Santorum said, referring to Gingrich. "There's too much about that candidate that we don't want to have" in a race that must focus on Obama's record, he said. "And we have another candidate who is just too darn cold, who doesn't have bold plans," Santorum said, alluding to Romney.

Romney, whose lead has shrunk in the race's closing days, opened Friday with fresh endorsements from Virginia Gov. Bob McDonnell and three House members from Texas who lined up with him now that Perry is out of the race. Romney was making stops Friday along the South Carolina coast, in the state's midlands and conservative north. Gingrich, buoyed by Perry's endorsement, concentrated on heavily pro-military Charleston area.

A day after questions about Gingrich's second marriage arose, Gingrich's third wife, Callista, was front and center when the couple appeared at The Medical University of South Carolina Children's Hospital. Mrs. Gingrich read her book, "Sweet Land of Liberty," to six children in a hospital play area as her husband watched from the sidelines and chatted with pediatricians.

Earlier, Gingrich scrapped an appearance at the Southern Republican Leadership Conference due to what campaign aides said was poor attendance. Conference organizers blamed a scheduling conflict.

The libertarian-leaning Ron Paul, whose support has slipped with his light campaign effort here, went ahead with his address to the Southern Republican group and said Saturday's primary could be a "significant event" that will help propel his insurgent campaign forward. He also warned voters not to back candidates who support the status quo and who won't make deep cuts to federal spending.

DeMint, appearing on CBS' "This Morning," predicted that Saturday's victor "is likely to be the next president of the United States."

Romney seemed to agree. His campaign released a new web ad with the tagline: "On Saturday South Carolina Picks a President." The ad included words of praise for him from South Carolina Gov. Nikki Haley and Arizona Sen. John McCain, the party's 2008 nominee.

Perry, having struggled in vain to build support in his native South, quit the race on Thursday 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 debate, 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. Later, he noted his 42-year marriage to wife, Ann.

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 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."