Mitt Romney said the Republican race is a neck-and-neck contest with a resurgent Newt Gingrich on the eve of the key vote in South Carolina, the state where Romney dropped out of the U.S. presidential race four years ago.

Romney, the former Massachusetts governor, has seen his lead narrow in a boisterous week that has given surprising boosts to more conservative rivals Rick Santorum and Gingrich, who is closing in despite new allegations from an ex-wife that he had asked her to share him with his mistress.

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

The entire field, including fourth and final candidate Ron Paul, scrambled Friday for the shreds of support left by Texas Gov. Rick Perry, who quit the race Thursday and endorsed Gingrich, a former speaker of the House of Representatives.

Birther redux

A judge in Atlanta, Ga., has ordered President Barack Obama to appear in court next week for a hearing on a complaint from a Georgia resident that says Obama isn't a natural-born citizen and can't hold the office.

It's one of many such lawsuits that have been filed across the country, so far without success. The complaint is intended to keep Obama's name off the state's ballot in the March presidential primary. There was no word on whether Obama will attend the hearing, which is set for Thursday.

Perry's departure, a raucous Thursday night debate and the fresh reminders of Gingrich's tumultuous personal life promised to make the dash to Saturday's voting frenetic in the fight for the chance to face President Barack Obama in November. The economy is the top issue in this election, and Obama oversees a frustrated country still trying to recover from the Great Recession.

Vying for momentum

If Romney wins in South Carolina, he could regain the momentum he had after winning this month's New Hampshire primary and finishing in a virtual tie in Iowa with Santorum, a former Pennsylvania senator. Saturday's vote also could determine whether Gingrich or Santorum emerges as his main rival.

Romney now clings to a narrow lead in polls in South Carolina, the first Southern state to vote in the nominating process and one that is rich with conservative and evangelical voters.

Romney remains the lukewarm leading candidate whose past stances on abortion and other social issues have failed to win the passion of the party's more conservative members. Gingrich is fighting Santorum for that support, though his ex-wife's reminder of his multiple marriages and affairs could hurt him, especially in South Carolina.

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U.S. Republican presidential candidates at a debate in Charleston, S.C. on Thursday. (Matt Rourke/Associated Press)

Gingrich cancelled a planned Friday morning appearance at the Southern Republican Leadership Conference due to poor attendance, with only two dozen people in the hall. He and his third wife, Callista, later appeared at an event at a children's hospital, just hours after ABC's "Nightline" showed Marianne Gingrich saying Gingrich conducted an affair with Callista "in my bedroom in our apartment in Washington" while she was elsewhere.

Gingrich has angrily denounced the news media for putting his ex-wife front and centre in the final days of the race. "Let me be clear, the story is false," he said at Thursday's debate.

Saturday could indicate whether voters can accept Gingrich's past, as Perry did in his endorsement Thursday: "Newt's not perfect, but who among us is," he said. "The fact is, there is forgiveness for those who seek God."

Tone of the race

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

Meanwhile, Santorum continues to portray himself as the party's true conservative and is trying to ride on the momentum of Thursday's surprise news that he led Romney by 34 votes in the final count of the leadoff Iowa caucuses. Romney initially had been declared the winner.

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U.S. presidential primary candidate Rick Santorum arrives for a campaign rally at Hudson's Smokehouse in Lexington, S.C. on Friday. (Benjamin Myers/Reuters)

"One candidate is too radioactive, a little too hot," Santorum told an audience Friday, 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.

Santorum added he's finally drawing enough campaign contributions to compete aggressively in Florida, the next primary state, and beyond, even if he finishes poorly in South Carolina.

Romney on Friday 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.