NORTH CHARLESTON, S.C - Mitt Romney said the Republican race is a neck-and-neck contest with a resurgent Newt Gingrich on the eve of the key primary 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 a surprising boost to Gingrich, who is closing in despite new allegations from an ex-wife that he had asked her to share him with his mistress.

If Romney wins in South Carolina, traditionally a make-or-break state for Republican candidates, he could regain the momentum he had after winning this month's New Hampshire primary and finishing in a virtual tie in Iowa with former Sen. Rick Santorum. 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.

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

The entire field, including Santorum in third place and Rep. Ron Paul in last, 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.

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.

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.

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."

Romney urged Gingrich to release a more detailed accounting of the investigation into his ethical problems during his tumultuous time 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.

In January 1997, Gingrich became the first House speaker ever reprimanded and fined for ethics violations, slapped with a $300,000 penalty. Gingrich admitted he'd failed to follow legal advice concerning the use of tax-exempt contributions to advance potentially partisan goals.

Gingrich's campaign countered that a vast amount of information from the ethics investigation had been public for more than a decade and said in a statement that Romney's campaign was "on a panic-attack."

Gingrich released his income tax records during the course of Thursday's 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.