Romney, who has made an issue of Gingrich's consulting work for Freddie Mac, shot back that his investments were made by a blind trust and were in mutual funds, not stock.
"But what the speaker did was to work as a spokesman to promote Fannie Mae and Freddie Mac, to protect them. . . . He got paid $1.6 million for that," Romney said.
Rick Santorum, the former Pennsylvania senator, said it was time to stop the "petty personal politics" over trivial issues and urged a change of subject.
"Can we set aside that Newt was a member of Congress and used the skills that he developed as a member of Congress to go out and advise companies - and that's not the worst thing in the world - and that Mitt Romney is a wealthy guy because he worked hard? You guys should focus on the issues."
The two-hour debate, the 18th in a series whose twists and turns have dominated the GOP nominating contest, was broadcast nationally on CNN. This time, there was a live audience in the hall at the University of North Florida.
Gingrich has fed off the live crowds at debates, and he seemed to miss the feedback in a match Monday night, when host NBC required attendees to refrain from applauding. Romney's campaign has expressed annoyance with the reality-show atmosphere of most of the debates, in which crowds of GOP activists jeer at the gladiators.
Santorum and libertarian Texas Rep. Ron Paul, who also debated, have trailed in the Florida polls; the focus Thursday was on the increasingly personal match between Gingrich and Romney.
In the debate, Romney accused Gingrich of "repulsive" and "over-the-top rhetoric" in a Spanish-language radio ad that had labeled him "anti-immigrant." Gingrich pulled the ad on Wednesday after Sen. Marco Rubio (R., Fla.) said it was offensive. Romney demanded an apology for what he called a "highly charged epithet."
Gingrich did not apologize.
Gingrich has been pushing to ride the momentum from his huge win in South Carolina last weekend, and Romney is clawing to get back on top of the nominating contest that, until recently, he seemed to be in control of despite misgivings about him among many conservatives.
Romney and his allies have been blanketing the airwaves in Florida with advertisements pounding Gingrich for deviations from conservative orthodoxy, ethics charges that led to a $300,000 settlement and that clouded the end of his tenure as House speaker in the late 1990s, as well as Gingrich's post-congressional work with Freddie Mac.
To the Romney campaign, Gingrich was an "influence peddler" in the classic Washington manner; Gingrich said he was paid for his expertise as a historian.
The most recent polls in Florida show Romney and Gingrich in a dead heat, though there was some indication the barrage from Romney had begun to slow the insurgent's momentum.
A CNN/Time poll released Wednesday showed Romney leading Gingrich 36 percent to 34 percent among likely primary voters, within the survey's margin of error. Santorum was drawing 11 percent to Paul's 9 percent, with 7 percent undecided.
The same day, a Quinnipiac University survey also showed Romney with 36 percent to Gingrich's 34 percent.
Earlier Thursday, Gingrich went on a populist rant against Romney at a tea party rally in the central Florida town of Mount Dora, telling a crowd of about 1,500 that his rival's attacks on him were "the last desperate stand of the old order." Gingrich was especially incensed at Romney's ads on the Freddie Mac contract, calling them dishonest and "grossly hypocritical" because the former Massachusetts governor owns investments in the mortgage giant and its sister, Fannie Mae.
"We're not going to beat Barack Obama with some guy who has Swiss bank accounts, Cayman Island accounts, owns shares of Goldman Sachs while it forecloses on Floridians and is himself a stockholder in Fannie Mae and Freddie Mac while he tries to think the rest of us are too stupid to put the dots together to understand what this is all about," Gingrich said at the rally.
He ducked a chance to defend that attack in the debate, telling moderator Wolf Blitzer it was an unworthy topic.
"Wouldn't it be nice if people didn't make accusations somewhere else that they weren't willing to defend here?" Romney said. He said he paid all the taxes he legally owed.
"I have earned the money that I have," he said. "I didn't inherit it. I take risks. I make investments."
In a sarcastic news release, the Romney campaign blasted Gingrich as a "Dr. Newt and Mr. Hyde" character who sounded like he was at an Occupy Wall Street rally with his attacks on capitalism. In mailings and other attacks, Gingrich was portrayed as a failed speaker whose ideas are "off the wall."
Contact politics writer Thomas Fitzgerald at 215-854-2718, email@example.com, or @tomfitzgerald on Twitter. Read his blog, "The Big Tent," at www.philly.com/BigTent.