Republican presidential candidate, former Pennsylvania Sen. Rick Santorum speaks to the media at a campaign stop at Valley High School, Tuesday, Jan. 3, 2012, in West Des Moines, Iowa. (AP Photo/Charlie Riedel)
CHARLESTON, South Carolina (CNN) -- Arguing that momentum was on his side after a delayed announcement of victory in Iowa, former Sen. Rick Santorum sought to weaken rivals Mitt Romney and Newt Gingrich with a series of direct attacks on the two front-runners in the last South Carolina debate ahead of the state's primary on Saturday.
Santorum charged both Romney and Gingrich would be weaker opponents against President Barack Obama, saying neither could draw a contrast with the incumbent over health care policy.
The former Pennsylvania senator called the Massachusetts health care plan passed while Romney was governor "an abject disaster" and said Gingrich has supported an individual mandate for 10 years.
"I've been fighting for health reform, private sector, bottom up, the way America works best, for 20 years. While these two guys were playing footsies with the left, Santorum said.
The debate came at the end of a ground-shaking day that saw Santorum being declared winner of the Iowa caucuses more than two weeks after his apparent second-place finish to Romney and Texas Gov. Rick Perry dropping out of the race and throwing his support to Gingrich. It is the last matchup between the four candidates two days before Saturday's pivotal first in the South primary.
Earlier an angry Newt Gingrich chastised CNN for starting off with a question about reports that he offered his then-second wife a choice of an open marriage or divorce when he revealed to her he was having an affair with the woman he later made his third wife.
"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 told CNN Chief National Correspondent John King, the moderator of the debate.
Gingrich's response elicited loud applause from the audience.
Hours before the debate, ABC News and The Washington Post released interviews with Marianne Gingrich in which she alleged Gingrich asked for an "open marriage" before their divorce.
Marianne Gingrich told The Washington Post that a day after he husband told her about his affair with Callista Bisek in May 1999, the former House speaker delivered a speech titled "The Demise of American Culture" to a group of Republican women in Pennsylvania.
"How could he ask me for a divorce on Monday and within 48 hours give a speech on family values and talk about how people treat people?" Marianne Gingrich said in her interview with the Post.
Gingrich said the story was false.
The three other presidential candidates on the stage passed on the chance to criticize Gingrich over the allegations by his ex-wife.
As the debate was going on, Gingrich's campaign released his tax returns as he had promised to do earlier. Romney was asked if he would follow his father's lead in releasing 12 years of tax returns.
"Maybe," Romney said, before highlighting his business experience and his personal success. "I didn't inherit money from my parents. What I have, I earned."
Santorum said he filed his own returns and they were at home on his computer and he would release them once he returned home. Paul said he had no intention to release his returns, arguing that congressional financial statements are revealing enough.
The issue of electability was also hotly debated. Earlier this week, Gingrich called for both Perry and Santorum to drop out of the race and support his candidacy because they didn't have the experience for a large undertaking like a presidential campaign. With Perry gone, Santorum was left to respond.
"Grandiosity has never been a problem with Newt Gingrich," Santorum said, arguing that he's the steady choice and Republicans shouldn't pick a nominee who might surprise them.
While Gingrich and Santorum debated their records in Washington and specifically who was more responsible for the Republican Revolution of the 1990s, Romney raised his hand to argue it's time for a Washington outsider.
"We need to send to Washington someone who has not lived in Washington, but someone who's lived in the real streets of America," Romney responded.
Every candidate onstage said they opposed the anti-Internet piracy bill roiling Congress, and criticized the proposal for being too intrusive. However, both Romney and Santorum said they could support a more narrow law that would help protect intellectual property rights online.
Displaying the off-the-cuff wit he has used to great effect at a series of GOP debates, Gingrich joked he was none too sympathetic to movie and music producers who pushed for the legislation.
"You're asking a conservative about the economic interest of Hollywood," Gingrich said to laughter in the audience. "And I'm weighing it, I'm weighing it. I'm not rushing in."
Meanwhile Paul touted his libertarian-leaning views, adding he had been an early detractor of the bill. He said as president he would be unique in his ability to work with leaders on both sides of the aisle to protect civil liberties.
Four polls over the last two days show Romney's long-held lead in South Carolina nearly evaporating into single digits by the time the four took the stage, while Gingrich has surged into a virtual tie. Gingrich on Thursday grabbed a major endorsement when Perry announced he was backing Gingrich as he suspended his campaign.
Santorum's campaign got a boost on Thursday when the Iowa GOP released the final certified results from its January 3 caucuses, which reversed Romney's eight-vote victory and instead determined that Santorum had won by 34 votes.
And Rep. Ron Paul of Texas had a solid third-place finish in Iowa and a strong second-place finish last week in New Hampshire, but most political pundits saw his performance in Monday's debate in Myrtle Beach, South Carolina, as strained.
Gingrich had rave reviews from his performance in Monday night's showdown, when he repeatedly went after Romney. And attacks by his rivals seemed to put the front-runner on his heels.
Gingrich has been criticized by fellow Republicans for his attacks on Romney's venture capitalist past, which some were uncomfortable with because they saw it as an attack on the free market and thought it could be damaging in the general election.