Republican presidential contenders Mitt Romney and Newt Gingrich clashed repeatedly in heated, personal terms Monday night in a crackling campaign debate, the former Massachusetts governor tagging his rival as 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 two-hour debate marked by occasional interruptions and finger-pointing.

The event 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.

His double-digit victory reset the race to pick a rival to challenge Democratic President Barack Obama this fall, and the next contest is the Jan. 31 Florida primary.

It is a state Romney can ill afford to lose, and he 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 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 retorted 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, referring to the $300,000 that Gingrich's consulting firm received in 2006 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 expressed pride in having supported it. "It has saved lives. It's run on a free enterprise model," he said in a state that is home to millions of seniors.

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 jumped at the chance to criticize both Romney and Gingrich for having supported the big federal bailouts of Wall Street in 2008.

He also said both men had abandoned conservative principles by supporting elements of "cap and trade" legislation to curb pollution emissions from industrial sites. "When push came to shove, they were pushed," he said.

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.

Paul has said he will largely bypass Florida to concentrate on states that are holding caucuses.

Hit at the outset with Romney' charge that he had resigned Congress in disgrace and went on to a career peddling his own influence, Gingrich said two men who had run against the former governor in the 2008 campaign, John McCain and Mike Huckabee, had said he couldn't tell the truth.

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.

Romney began airing a harshly critical new campaign ad and said the former House speaker had engaged in "potentially wrongful activity" with the consulting work he did after leaving Congress in the late 1990s.

Gingrich retorted that Romney was a candidate who was campaigning on openness yet "has released none of his business records."

He followed up two hours before the debate by arranging the release of a contract his former consulting firm had with the Federal Home Loan Mortgage Corp. for a retainer of $25,000 per month in 2006, or a total for the year of $300,000. The agreement called for "consulting and related services."

Despite Romney's attempts to call Gingrich a lobbyist, the contract makes no mention of lobbying.