Mitt Romney and Newt Gingrich clashed sharply and repeatedly in a Republican presidential debate here Monday night, with the former Massachusetts governor attacking the former House speaker as a failed leader, a K Street influence peddler and a candidate who would put the party at risk in the general election.

Romney, seeking to regain the offensive after a loss in the South Carolina primary on Saturday that broke the GOP race open, adopted a far different demeanor than he displayed during two lackluster debates last week. In Monday’s forum, he kept the pressure on Gingrich, particularly in the early stages, and he tried to raise doubts about his opponent’s ability to lead the party.

“You spent now 15 years in Washington on K Street,” Romney said about the former speaker’s years after he left Congress. “And this is a real problem, if we’re going to nominate someone who not only had a record of great distress as the speaker but that has worked for 15 years lobbying.”

Gingrich, whose pugnacious debating style helped propel him to victory in South Carolina, was far more subdued as he parried Romney’s attacks. He accused the former governor of getting his facts wrong and making distorted charges, and he predicted that the voters would reject such politics.

Gingrich protested when Romney said the former speaker had lobbied members of Congress over legislation to add a prescription drug benefit to Medicare. “You just jumped a long way over here, friend,” he said sarcastically. “Let me be very clear, because I understand your technique. . . . It’s not going to work very well, because the American people see through it.”

The debate came at a critical time in the Republican presidential race, with Romney’s candidacy suddenly at risk and with Gingrich unexpectedly in strong contention for the party nomination.

Two other candidates were on the stage — former senator Rick Santorum (Pa.), who won the Jan. 3 Iowa caucuses, and Rep. Ron Paul (Tex.) — but the focus now is almost entirely on Romney and Gingrich, and the tenor of the debate reflected that political reality. Far more than in some past forums, the two front-runners spent more time challenging each other than trying to make a case against President Obama or describing their plan to turn around the economy.

Although the exchanges between Romney and Gingrich were pointed, the debate, held at the University of South Florida, lacked the carnival atmosphere of those in South Carolina, where audiences cheered loudly and egged on the candidates.

The debate came as Romney, bowing to pressure, released his 2010 tax returns and estimates for 2011. He acknowledged Sunday that his campaign had erred by not taking action when the issue first arose.

Earlier in the day, Romney demanded that Gingrich release his contract with housing giant Freddie Mac, which paid the former speaker’s firm about $1.6 million over a period of years. Shortly before the debate, Gingrich posted one of those contracts on his campaign Web site, showing an annual payment of $300,000.

But Romney wasn’t prepared to let the issue go as he mocked his rival’s claim in an earlier forum that he was hired as a historian by the agency and noted that Freddie Mac’s chief lobbyist brought him onboard.

“If you read the contract . . . it says very clearly supposed to do consulting work,” Gingrich responded. “The governor did consulting work for years. I have never suggested his consulting work was lobbying. . . . There is no place in the contract that provides for lobbying. I have never done any lobbying.”

When Romney also charged that Gingrich had lobbied members of Congress over a Medicare drug provision, the former speaker protested.

“I’m proud of the fact that I publicly, openly advocated Medicare Part D,” he said. “It has saved lives. It’s run on a free-enterprise model.”

Romney countered that one of Gingrich’s companies was representing health-care companies that stood to benefit from passage of the legislation.

“You can call it whatever you’d like,” Romney said. “I call it influence peddling.”

The issue of Gingrich’s tenure as House speaker also figured prominently early in the debate as the candidates discussed who was more electable.

“I think it’s about leadership, and the speaker was given an opportunity to be the leader of our party in 1994,” Romney said. “And at the end of four years, he had to resign in disgrace.”

Gingrich described his four years as speaker as one of significant accomplishments, including a balanced budget, welfare reform and the reelection of a Republican Congress.

“I left the speakership after the 1998 election because I took responsibility for the fact that our results weren’t as good as they should be,” he said. “I think that’s what a leader should do.”

He said his record of building the Republican Party, from helping win the House in 1994 to maintaining those majorities while he was speaker, was far superior to anything Romney could point to as a governor and as head of the Republican Governors Association.

When Romney noted the House reprimand of Gingrich for ethics violations, Gingrich said he had asked Republicans to vote for it to get the issue behind him and the GOP-controlled House.

Paul leapt in to support Romney’s assessment of Gingrich’s speakership, saying: “It was chaotic.”

Paul also disputed Gingrich’s contention that stepping down from the speakership was an act of noble leadership following the GOP’s disappointing electoral results in 1998.

“He didn’t have the votes,” Paul said. “That was what the problem was. So this idea that he voluntarily reneged and he was going to punish himself because we didn’t do well in the election, that’s just not the way it was.”

The issue of Romney’s time at Bain Capital came up several times, with Romney again lamenting that some of his rivals had picked up the “tools of the left” to advance their candidacies in the Republican primaries.

When Santorum was asked about that, he demurred — at first. He said he had never criticized Romney’s work at Bain but then leveled a criticism at both the front-runners. He questioned whether Romney or Gingrich is as committed to capitalism as they have argued on the campaign trail, noting that both supported the federal bailout of Wall Street in 2008.

“My question to Governor Romney and to Speaker Gingrich: If you believe in capitalism that much, then why did you support the bailout of Wall Street, where you had an opportunity to allow destructive capitalism to work, to allow a failure of a — of a system that needed to fail because people did things that in capitalism pay, you pay a price?”

Monday’s debate ended a day of escalating attacks by Romney and a sharp rebuttal from Gingrich.

When Romney demanded that Gingrich release records from his work as a consultant for Freddie Mac and other entities, as well as documents related to the ethics investigation, he warned that they could reveal “potentially wrongful activity.” But neither he nor his advisers could cite anything specific that led the candidate to make such a statement.

Gingrich, campaigning in the Tampa area, dismissed it as the sign of someone who considers his candidacy sinking fast in the aftermath of his loss in South Carolina. “It’s such baloney,” he said at a rally. “It used to be pious baloney, now it’s just desperate baloney.”

After spending all last week on the defensive, Romney hoped to turn the tables on Gingrich. His onetime momentum now gone, Romney is counting on his superior organization and financial head start to gain an edge in Florida, which will hold its primary next Tuesday.

Romney already has a head start on the airwaves. His super PAC, Restore Our Future, has spent $5 million in Florida, according to Federal Election Commission filings. And his campaign has been on the air for three weeks.

Gingrich is working overtime to catch up. On the heels of his South Carolina victory, his campaign set a goal of $1 million for an online “money bomb” but quickly raised it to $2 million over two days. And the PAC backing Gingrich, Winning Our Future, has set a goal of spending $10 million in the Florida primary.

Miriam Adelson — the wife of casino and hotel mogul Sheldon Adelson, whose $5 million donation helped make Gingrich competitive in South Carolina — has promised a Gingrich super PAC another $5 million by this week, a source close to Adelson said. The news of the Adelson donation was first reported by the Las Vegas Sun.

Staff writers Amy Gardner and James V. Grimaldi in Washington and Nia-Malika Henderson and Philip Rucker in Tampa contributed to this report.