With multiple polls showing Gingrich's support among Republicans on the rise as Romney's backing fades, the former Massachusetts governor used the debate in Tampa to highlight Gingrich's 1998 resignation as U.S. House speaker after an ethics investigation. Romney called Gingrich an "influence peddler in Washington" during his post-congressional career advising companies on legislative matters.
"The speaker was given the opportunity to be the leader of our party in 1994, and after four years he resigned in disgrace," Romney said.
Gingrich, while calling Romney desperate, said the attacks on his record as speaker and in the private sector were inaccurate, The New York Times reported.
The Romney-Gingrich back-and-forth dominated the first hour of the debate, with the other two contenders for the GOP nomination -- Rep. Ron Paul of Texas and former U.S. Sen. Rick Santorum -- looking on, the newspaper said.
Romney began painting Gingrich as a failure and a fraud, and Gingrich said Romney is inauthentic and politically vulnerable, as a poll indicated Gingrich maintained his momentum after his South Carolina primary win Saturday, leading Romney 34 percent to 26 percent in the Sunshine State.
The Insider Advantage poll of 557 Republicans showed Paul with 13 percent support and Santorum with 11 percent.
Paul leads all GOP candidates in Florida with 30 percent support among voters age 30 to 44. But Gingrich leads Paul among voters age 18 to 29, the poll conducted Sunday for conservative Florida news outlet Newsmax Media found.
The poll has a margin of error of 4 percentage points.
Romney, who came in second in South Carolina Saturday, attacked Gingrich at a Sunday evening rally in Ormond Beach, Fla., ahead of the state's Jan. 31 primary.
"We're not choosing a talk show host -- we're choosing the person who should be the leader of the free world," Romney said.
He then brought up a 15-year-old ethics investigation in which Gingrich, as House speaker, was disciplined by his colleagues for ethics violations.
"It was proven that he was a failed leader, and he had to resign in disgrace," Romney said. "I don't know whether you knew that. He actually resigned after four years, in disgrace."
Eighty-four ethics charges were filed against Gingrich during his 1995-1999 term as speaker. After an extensive investigation and negotiation by the House Ethics Committee, Gingrich was reprimanded and fined $300,000 by a 395-28 House vote. It was the first time the House ever disciplined a speaker for ethical wrongdoing.
Most of the charges were later dropped. One not dropped was a charge of claiming tax-exempt status for a college course run for political purposes.
Gingrich told CNN's "State of the Union" Sunday the $300,000 he paid was actually a reimbursement of the House investigation's costs. He said all 1,300 pages of the ethics report were available online.
On the CBS News program "Face the Nation" Gingrich belittled Romney as fake and politically open to attack because of his shifts away from more liberal positions he held as Massachusetts governor.
"Gov. Romney's core problem was that [as] the governor of Massachusetts [he was] moderate, which by the standards of Republican primary voters is liberal. And he can't -- he can't relax and be candid," he said.
"He's been dancing on eggs trying to figure out how to find a version of Romney that will work," Gingrich said.
Gingrich told supporters on Twitter Sunday he had an "amazing 24 hours" raising more than $1 million and attracting "1000s of volunteers & momentum going forward."
Romney told Twitter followers: "Don't wait until Jan 31st. Cast your vote early. Click here to find out where to cast your ballot."
On television, Romney addressed the controversy around his income-tax returns.
He told "Fox News Sunday" he would release his 2010 return and an 2011 estimate Tuesday, reversing his earlier position he would release his returns in April.
He acknowledged his previous reluctance was a "mistake."
Both candidates -- along with Santorum and Paul -- are to participate in the first Florida debate at 9 p.m. Monday.
The televised debate, at the University of South Florida in Tampa, is sponsored by the Tampa Bay Times, NBC News, the National Journal and the Florida Council of 100, a non-partisan business group that advises Florida's governor.
"The inevitability of Romney has now been wiped away," Santorum told The Washington Post after a rally in Coral Springs, Fla.
He called Gingrich an unreliable conservative.
"He didn't live up to all the hype," Santorum said. "It's great to be glib. It's better to be principled."
Paul will participate in the debate but will largely bypass the Florida primary to focus on smaller February caucuses, such as Nevada, where he has a steadfast following. This decision was announced before the Newsmax poll was released.