In a forceful attack, Minnesota Rep. Michele Bachmann said Gingrich "had his hand out and received $1.6 million to influence senior Republicans and keep the scam going in Washington, D.C.," for Freddie Mac, a government-backed housing entity.
"Just not true," Gingrich shot back. "I never lobbied under any circumstances," he added, denying an allegation she had not made.
The clash underscored the state of race, with Gingrich, the former House speaker, atop the polls in Iowa and nationally, while Texas Rep. Ron Paul, former Massachusetts Gov. Mitt Romney and his other pursuers work in television ads and elsewhere to overtake him in the final days before the caucuses.
Former Sen. Rick Santorum of Pennsylvania, who has staked his campaign on Iowa, was quick to challenge Gingrich as a conservative leader. He recalled that Gingrich had to contend with a "conservative revolution' from the ranks of Republican lawmakers when he was House speaker in the 1990s.
Romney, who runs second in the polls in Iowa, largely refrained from criticism Gingrich, despite increasingly barbed attacks in day-to-day campaigning. Instead, he firmly rejected suggestions that he had once favored gay marriage only to switch his position. "I have been a champion of protecting traditional marriage," he said.
Given the stakes, Gingrich, Bachmann and Santorum weren't the only contenders eager to impress Iowa voters and a nationwide television audience with their conservative grit.
"I hope I am the Tim Tebow of the Iowa caucuses," said Texas Gov. Rick Perry, referring to the Denver Broncos quarterback whose passing ability draws ridicule but who has led his team to a remarkable seven wins in eight weeks.
"We're getting screwed as Americans," said former Utah Gov. Jon Huntsman, insisting that he, in fact, was a steadier conservative than any of the others on stage.
"Anybody up here could beat Obama," said Rep. Ron Paul of Texas, whose views verge on libertarianism and who has struggled to expand his appeal.
And Bachmann, who was quicker than any other candidate to criticize a rival, bristled when challenged repeatedly on the accuracy of her facts. "I am a serious candidate for president of the United States, and my facts are accurate," she said.
Indeed, the big question in the opening moments of a fast-paced two-hour debate went to the heart of a dilemma that could eventually settle the race - do conservative Republican caucus and primary voters pick a candidate with their hearts, or do they look elsewhere if they judge their favored candidate might not be able to defeat the president.
Those voters begin making that choice on Jan. 3, and if experience is any guide, one or more of the presidential hopefuls on the debate stage will not make it out of the state to compete in the New Hampshire primary a week later.
Gingrich, who seemed an also-ran in the earliest stages of the race, has emerged as a leader heading into the final stretch of the pre-primary campaign.
His decades in Washington and his post-congressional career as a consultant have been the subjects of tough critiques from Romney's campaign in the past week.
But the former speaker passed up an offer to criticize his rival on the issue of Medicare, saying, "I'm not in the business of blaming Gov. Romney." In fact, he said, Romney has made constructive suggestions for preserving the program that tens of millions of Americans rely on for health care yet faces deep financial woes.
Gingrich drew criticism earlier in the year for calling a GOP Medicare proposal "right-wing engineering." Romney refrained from criticizing that plan but did not embrace it in full.
Bachmann, who has long-since faded to the back of the pack in the polls, showed no such reluctance.
When he labeled her charges inaccurate, she shot back that when she made similar contentions in the previous debate, she was judged factually accurate by an independent arbiter. She said Gingrich's work for Freddie Mac was in furtherance of a "grandiose scam" to keep alive an entity at the heart of the housing crisis.
"I will state unequivocally for every person watching tonight: I have never once changed my positions because of any payment," Gingrich said, adding that in fact, he favored breaking up both Fannie Mae and Freddie Mac, his benefactor.
Moments later, Bachmann challenged Paul even more aggressively, saying his refusal to consider pre-emptive action to prevent Iran from getting a nuclear weapon were dangerous.
"The problem would be the greatest under-reaction in world history if we have an avowed madman who uses that nuclear weapon to wipe nations off the face of the earth," she said, referring to an International Atomic Energy Agency report that said Iran was "within just months of being able to obtain that weapon."
Paul questioned the report. "They have no evidence; there has been no enrichment," he said.