The former Massachusetts governor called Gingrich a Washington "influence peddler," only to be accused in turn of spreading falsehoods over many years in politics.
"You've been walking around the state saying things that are untrue," Gingrich told his rival in a crackling two-hour debate.
Former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul rounded off the four remaining GOP contenders.
Gingrich won the South Carolina primary in an upset last weekend. His double-digit win reset the race to pick a rival to challenge Democratic President Barack Obama this fall.
The Florida primary is on Jan. 31.
Following his defeat in South Carolina, Romney was the aggressor from the opening moments Monday night. He said Gingrich had "resigned in disgrace" from Congress after four years as speaker and then had spent the next 15 years "working as an influence peddler."
In particular, he referred to the contract Gingrich's consulting firm had with Freddie Mac, a government-backed mortgage giant. Calling Gingrich a lobbyist, Romney said it "did a lot of bad for a lot of people and you were working there."
"I have never, ever gone and done any lobbying," Gingrich shot back emphatically, adding that his firm had hired an expert to explain to employees "the bright line between what you can do as a citizen and what you do as a lobbyist."
When Gingrich sought to turn the tables by inquiring about the private equity firm that Romney founded, the former Massachusetts governor replied: "We didn't do any work with the government. ...I wasn't a lobbyist."
When the debate turned to immigration, one moderator noted that Romney and Santorum have said they would veto the "Dream Act," which would create conditions under which illegal immigrant minors might achieve U.S. citizenship, and asked if Gingrich agreed.
"No, I would work to get a signable version," he said. "I think any young person brought here by their parents when they were young should have the same opportunity to join the American military and earn citizenship."
Romney said that was the same as his position.
Moments later, he was asked to reconcile two other statements he has made about immigration, that while he doesn't want to deport millions of illegal immigrants, he wants them to return to their home countries and apply for citizenship. "The answer is self-deportation, which is people decide they can do better by going home," he said.
At times, the other two contenders on stage were reduced to supporting roles.
Asked if he could envision a path to the nomination for himself, Santorum said the race has so far been defined by its unpredictability.
The Associated Press contributed to this report.