Updated 10:07 p.m.

Mitt Romney and Newt Gingrich, each battling furiously for a win in Saturday's South Carolina primary, pointedly questioned each other's experience to be president, while being forced to account for standing questions about the challenges they face to winning the nomination and beating President Obama.

The two leading candidates in South Carolina's primary this weekend largely avoided sniping at each other in the first half hour of the debate -- a spirited affair less than 36 hours before voting begins in the Palmetto State -- but engaged each other more directly as the evening progressed.

Gingrich was pressed to explain his past support for a mandate for individuals to purchase health insurance, and his manner of leadership as speaker of the House, a tenure described by critics as erratic.

But Gingrich scored early -- and decisively -- with a fiery response to allegations from an ex-wife that drew wild applause from the crowd in attendance.

Romney, meanwhile, had to defend his business record and answer questions as to why he wouldn't release his income tax records, all while relitigating conservative criticism of the health care reform he signed as governor of Massachusetts.

Through this, former Pennsylvania Sen. Rick Santorum, crowned the winner this morning of the Iowa Caucuses after a retabulation of results, was anxious to take on both Romney and Gingrich, distinguishing himself as a steady if not-flashy alternative to the two leading candidates.

The debate, the 17th of the cycle, followed one of the most dramatic days of the 2012 campaign. Thursday saw Texas Gov. Rick Perry’s exit from the campaign trail, new extramarital allegations against Gingrich, polling data showing Romney’s advantage slipping in South Carolina, and a new declaration by the Iowa GOP anointing former Santorum – not Romney – the winner of its Jan. 3 caucus after certifying official results.

Perry drops out of GOP presidential race, endorses Gingrich

The tone of the forum was set early when Gingrich angrily assailed CNN moderator John King for opening the debate by asking Gingrich to answer allegations made by his ex-wife, Marianne, in an interview with ABC News, saying the then-speaker of the House asked to engage in an "open marriage," or else he would file for divorce.

"I am appalled that you would begin a presidential debate with a topic like that," Gingrich said, earning wild applause from the audience. "To take an ex-wife and make it two days before the primary a significant question in a presidential campaign is as close to despicable as anything I can imagine."

Gingrich angrily rebuffs questions about ex-wife

Gingrich disputed the allegations as "false," and his three fellow Republicans onstage resisted piling on. ("Let's get on to the real issues," former Massachusetts Gov. Mitt Romney said.)

Recent pollng of the race suggests that Gingrich has been enjoying a late surge in South Carolina, one that could threaten Romney's bid for a win that, his campaign hopes, would all but seal the nomination for the former Massachusetts governor.

An NBC News-Marist poll released Thursday found Romney leading at 34 percent among likely primary voters in the state, followed by Gingrich at 24 percent, Texas Rep. Ron Paul at 16 percent, former Pennsylvania Sen. Rick Santorum at 14 percent, and Texas Gov. Rick Perry at 4 percent. But in the latter half of the two-day poll, following Gingrich's strong performance during a Monday debate, Romney's lead winnowed to five points.

NBC poll: Newt Gingrich gains ground on Mitt Romney in South Carolina
 
South Carolina has correctly predicted the eventual Republican nominee since the inception of its primary in 1980; in each subsequent contest, the winner has gone on to become the GOP standard-bearer.

Romney has sought to project an air of inevitability surrounding his candidacy, but has been dogged by questions about the business practices of Bain Capital, the private equity firm he cofounded, that go to the core of his argument that he is the candidate most experienced to repair the U.S. economy. Romney's work for Bain also made him wealthy, and Romney's GOP rivals have pressed him to release his tax returns.

He dealt with both issues Thursday evening. Romney sought to explain Bain's work in greater detail, highlighting instances in which its work created jobs. He avoided engaging with Republicans, like Gingrich, who have questioned Romney's private sector record.

"I'm someone who believes in free enterprise," he said. "And I'm going to stand and defend capitalism across this country, throughout this campaign. 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 also faced pressure to release his tax returns. He said he would release records -- going back an unspecified number of "multiple" years -- but not until April, by which time the primary may well be settled.

It was Santorum, though, who put the most pointed questions to the two frontrunners. Santorum, who served in Congress while Gingrich was speaker, raised questions about whether Gingrich's conduct as a leader would lead to a "worrisome moment" for the GOP.

"Grandiosity has never been a problem with Newt Gingrich. He handles it very, very well," Santorum said, later adding: "I knew what the problems were going on in the House of Representatives when Newt Gingrich was leading there. It was an idea a minute, no discipline, no ability to be able to pull things together."

That exchange opened up a broader, sharper discussion between the candidates on their backgrounds. Romney characterized Gingrich as a lifelong insider, and again touted his business experience as the best qualification for his candidacy.

"I was in business 25 years. So you're not going to get credit for my 25 years," Romney said. "I don't recall a single day saying, 'Oh, thanks heavens Washington is there for me.'"

But Romney was also put on the spot by Santorum, and later, Gingrich, over his record in Massachusetts. Gingrich accused Romney of continuing to support abortions even after having announced his opposition to abortion rights. And Santorum went on the attack on Massachusetts health care reform.

"It is not a free-market health care system. It is not bottom-up. It is prescriptive and government. It was the basis for Obamacare," Santorum said.

Romney stumbled at moments and offered wonky answers in response to the criticism, repeatedly vowing that, for whatever his past record shows, he would govern in opposition to abortion rights.

"I did my very best to be a pro-life governor. I will be a pro-life president," Romney said. 

Texas Rep. Ron Paul at times fell to the background, having to make quips at moment about not being afforded an opportunity to join the scrum onstage.  At one point, when the moderator was ready to move on after a question on abortion, the crowd complained that Paul hadn't been given an opportunity to answer.

The debate came after one of the most momentus days in the campaign. Perry ended his bid for the nomination and threw his support behind Gingrich, who has shown signs of revival in South Carolina, and who has sought to rally conservatives under the banner of being the best alternative to Romney.

That narrative shaped Thursday night's debate, which saw Gingrich engage in frequent crowd-pleasing answers, dropping references to Ronald Reagan and taking frequent shots at the media.

Whether Romney did much to reverse his slide likely won't be known until Saturday, when South Carolinians head to the polls. Debate settings have been a strength for Gingrich, and he, Paul and Santorum have relished the opportunity to pile on Romney in these settings. (Another debate is scheduled for Monday night in Florida.)

The debate, hosted by CNN and the Southern Republican Leadership Conference, was broadcast at 8 p.m. ET.