Former House speaker Newt Gingrich scolds a moderator at a debate in Charleston, South Carolina, on Thursday. [Jason Reed / Reuters]

CHARLESTON, South Carolina - US 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, a former speaker of the US House of Representatives, had perhaps the best shot.

Gingrich came under attack frequently during the debate of almost two hours.

But the fact that character issues were aired publicly, as well as sharp questions about his tenure as House speaker in the 1990s, 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 disclosed his own. Romney pledged to do so in April.

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

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

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."

His second wife, Marianne, told ABC News that Gingrich had sought an open marriage while having an affair with current wife Callista.

Reuters