Surging Republican White House hopeful Newt Gingrich blasted US media digging into his past marital woes and questioned rival Mitt Romney's job-creation claims as they clashed in a pivotal debate.

Gingrich, Romney, Christian conservative former senator Rick Santorum and veteran Representative Ron Paul fenced against each other and ripped President Barack Obama as they hunted for an advantage in South Carolina's Saturday primary.

Gingrich, a former House speaker hot on Romney's heels here, denounced the debate moderator for kicking off with a question about his second ex-wife's claim that she rejected his request for an "open marriage" and to tolerate an extra-marital affair.

"I am tired of the elite media protecting Barack Obama by attacking Republicans," he thundered, winning a standing ovation as he denied the story as "false" and called the question "as close to despicable as anything I can imagine."

Asked whether the issue was legitimate, Santorum celebrated Christian "forgiveness" but said voters should consider "issues of character" when they choose their nominee to take on Democrat Obama in the November elections.

Romney, a former Massachusetts governor and multi-millionaire investor, strove to turn aside attacks from Gingrich that he built his vast fortune while firing workers, saying he expected such jibes from Obama, not fellow Republicans -- traditionally the party of business.

"I know we're going to get hit hard from President Obama, but we're going to stuff it down his throat and point out it is capitalism and freedom that makes America strong," Romney said.

Gingrich charged that the approach of Romney's Bain Capital firm was to "take over a company and dramatically leverage it, leave it with a great deal of debt, (make) it less likely to survive."

Santorum joined in, stressing "we need a party that just doesn't talk about high finance and cutting corporate taxes or cutting the top tax rates" and saying Republicans must help "working men and women of this country who are out there paddling alone in America right now."

Romney, who has insisted he is the most electable Republican even as his once robust lead here has dwindled as Gingrich has surged, happily trained his fire on the Democratic incumbent he hopes to take on in November.

"This president is the biggest impediment to job growth in this country. And we have to replace Barack Obama to get America working again," he said.

Romney, the party establishment's favorite and the off-again, on-again frontrunner over the past year, hopes a convincing win here would let him wrap up the nomination.

But the frontrunner faced several setbacks on Thursday: Texas Governor Rick Perry dropped out of the race and endorsed Gingrich, raising the prospect that fragmented conservatives could rally around a single alternative to Romney.

And Iowa authorities rescinded Romney's eight-vote victory in the heartland state's first-in-the-nation caucus and said Santorum was the winner.

"The narrative that Governor Romney and the media have been touting of 'inevitability' has been destroyed," said Santorum's communications director, Hogan Gidley.

Romney has also faced criticism for refusing to make public his tax filings after saying he paid 15 percent of his income in taxes and said making $375,000 for nine speeches was "not very much."

At the debate, Romney said he would release his tax filings "in April" when his filings "are complete for this year" and indicated he might release several years.

It was unclear whether Gingrich would be able to sustain his momentum and defeat the frontrunner's more organized, better-funded campaign.

Santorum assailed Gingrich as a dangerously erratic leader and blasted Romney over his health care plan in Massachusetts, which Obama has called an inspiration for his own health care overhaul -- reviled by Republicans.

"It is an abject disaster. He's standing by it," the former senator said, pushing a line of attack cited by Romney critics who doubt his conservative credentials.

Turning to Gingrich, Santorum called him "a friend" but accused him of "grandiosity" and warned against picking a nominee who might be "going to pop."

The candidates still in the race after South Carolina will face off again in Florida, a vote-rich battleground that holds its primary January 31.