"You've been walking around the state saying things that are untrue," Gingrich said to his rival in a two-hour debate marked by interruptions and finger pointing.
The debate marked the first encounter among the four remaining GOP contenders - former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul shared the stage - since Gingrich won the South Carolina primary in an upset last weekend, a double-digit victory that reset the race to pick a rival to challenge President Barack Obama this fall.
Romney was the aggressor from the opening moments Monday night, saying 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 Washington.
In particular, he referred to the contract Gingrich's consulting firm had with Freddie Mac, a government-backed mortgage giant that he said "did a lot of bad for a lot of people and you were working there."
Romney also said Gingrich had lobbied lawmakers to approve legislation creating a new prescription drug benefit under Medicare.
"I have never, ever gone and done any lobbying," Gingrich said 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."
Romney counterpunched quickly, referring to what he said was more than $1 million that Gingrich's consulting firm received from Freddie Mac, the government-backed mortgage giant.
And 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"
As for the Medicare prescription drug benefit, Gingrich said that while he never lobbied lawmakers, as Romney charged, he was proud of having supported it. "It has saved lives. It's run on a free enterprise model," 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. He conceded he had been defeated for re-election in 2006 in Pennsylvania but said the party lost the governorship by an even bigger margin than his own defeat.
"There's one thing worse than losing an election and that's not standing for the principles that you hold," he said, a comment he frequently makes while campaigning in an attempt to question Romney's commitment to conservatism.
Paul sidestepped when moderator Brian Williams of NBC asked if he would run as a third-party candidate in the fall if he doesn't win the nomination. "I have no intention," he said, but he didn't rule it out.
The polls post-South Carolina show Gingrich and Romney leading in the Florida primary. That and the former speaker's weekend victory explained why the two were squabbling even before the debate began, and why they tangled almost instantly once it had begun.