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Matt Rourke, Associated Press
Callista Gingrich, wife of Republican presidential candidate, former House Speaker Newt Gingrich reads during a visit to Children's Hospital, Friday, Jan. 20, 2012, in Charleston, S.C.

NORTH CHARLESTON, S.C. — Mitt Romney sized up the 2012 presidential race in South Carolina as a neck-and-neck contest on the eve of the state's pivotal primary and insisted Friday that he's the one candidate Republicans can trust to "post up well" against President Barack Obama. He trained his criticism on Newt Gingrich, a sure sign of the momentum behind the former speaker's rise-and-fall-and-rise candidacy.

Rick Santorum and Ron Paul argued they were still in the mix as South Carolina's Sen. Jim DeMint declared the state a "two-man race." Santorum said he's finally drawing enough campaign contributions to compete aggressively in next-up Florida and beyond, even if he finishes poorly in South Carolina.

Romney, campaigning in Gilbert, acknowledged Gingrich's recent rise in the polls by singling him out as his chief competitor in the state Romney lost four years ago. In 2008, Romney abandoned South Carolina when it became clear he would lose big.

He called on Gingrich to release a more detailed accounting of the investigation into his ethical problems as House speaker, saying, "You know it's going to get out ahead of the general election." It was a sharp rejoinder to Gingrich's calls for Romney to quickly release his tax records.

Romney tried to frame a tight race as progress in the state he'd lost soundly before, although he's lost ground in recent polls.

"Frankly to be in a neck-and-neck race at this last moment is kind of exciting," he said

Romney called Gingrich "a feisty competitor" but argued the former House speaker was not the best man to put up against Obama. Surrogates to the former Massachusetts governor used a morning conference call with reporters to run down Gingrich's record on controlling government spending.

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 an 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.

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 supporters, in the call with reporters, said Gingrich oversaw 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."

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.

Gingrich, buoyed by Perry's endorsement, concentrated his efforts on the 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.

Gingrich is trying to move beyond new allegations from an ex-wife that he had sought an open marriage before their divorce.

Santorum played aggressor during a Thursday night debate, trying to inject himself into what seemed increasingly like a Romney-Gingrich race.

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 News Channel, said he didn't want to give Obama and the Democrats a "nice little present of having multiple releases." He said past GOP nominees McCain 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.