Republican presidential candidate Newt Gingrich angrily defended himself on Thursday against marital misconduct allegations at a raucous campaign debate that may determine whether he can pull off a surprise primary victory in South Carolina.

The debate was the final chance for rivals to chip away at front-runner Mitt Romney's lead in South Carolina and Gingrich, the former speaker of the U.S. House of Representatives, had perhaps the best shot.

Gingrich came under attack frequently in the debate of almost two hours and deftly defended himself with the kind of strong debate performance that has buoyed his up-and-down campaign all year.

But the fact that character issues were aired publicly, as well as sharp questions about his 1990s tenure as House speaker, could remind voters of the baggage Gingrich carries and damage his hopes.

In a move designed to embarrass Romney, Gingrich released his most recent tax returns, a reminder that the multi-millionaire Romney still had not produced his own. Romney pledged to do so in April.

I'm not going to apologize for being successful, said Romney.

Romney will take a huge step toward claiming the Republican nomination if he wins on Saturday after his New Hampshire win on January 10 and his near-victory in Iowa on January 3.

Romney's rivals had him in their sights to try to deny him victory, expressing doubts about his conservative convictions.

The CNN-sponsored debate, coming as South Carolina prepares to vote on Saturday, got off to a unpredictable start when moderator John King asked Gingrich to respond to charges put forth by his ex-wife Marianne that he had sought an open marriage while having an affair.

The impropriety charges have dogged Gingrich for years and threaten to slow his momentum in South Carolina as he seeks to upset Romney in the first primary vote in the South on Saturday.

I think the disruptive, vicious, negative nature of the news media makes it harder to govern this country, Gingrich fumed. I am appalled that you would begin a presidential debate on a topic like that.

GINGRICH GETS CLOSER

A strong performance in a debate in South Carolina on Monday helped Gingrich get within touching distance in the polls of Romney, who has struggled to explain why he has not released his tax forms.

But Gingrich has faced troubling questions that could halt his momentum. His second wife, Marianne, told ABC News that Gingrich had sought an open marriage while having an affair with current wife Callista. She said he should not be considered electable in the race to find a Republican challenger to Democratic President Barack Obama in next November's election.

A new Reuters/Ipsos poll conducted of 656 likely South Carolina voters showed Romney with 35 percent support, Gingrich with 23 percent support, and former Senator Rick Santorum with 15 percent support.

(Additional reporting By Sarah Irwin; Editing by Jackie Frank)