Mitt Romney painted rival Newt Gingrich as a Washington "influence peddler" and unfit for the White House, as falling poll numbers forced him onto the offensive in a fierce Republican debate.

Romney began the maiden Florida primary debate late Monday swinging hard at Gingrich, as he scrambled to get his campaign back on track and reassert himself as the candidate best able to beat President Barack Obama in November.

The contest was a marked role reversal from previous debates, when an insurgent Gingrich had railed against the frontrunner's past, portraying Romney -- a multimillionaire businessman -- as a ruthless corporate raider.

The tone was set earlier in the day by two new polls showing Gingrich surging past the former Massachusetts governor, just eight days before a vote that could either confirm Gingrich's surge or reinstate Romney as the favorite.

Facing a suddenly daunting hurdle to the nomination, Romney scrapped his single-minded focus on Obama, instead castigating Gingrich's past roles as speaker of House of Representatives and as a Washington lobbyist.

Romney zeroed in on Gingrich's work for state-backed mortgage lender Freddie Mac, a firm many Republicans see as being complicit in the housing bubble and the ensuing Great Recession, which hit Florida particularly hard.

"In the 15 years after he left the speakership, the speaker has been working as an influence peddler in Washington," Romney said.

"I don't think we can possibly retake the White House if the person who's leading our party is the person who was working for the chief lobbyist of Freddie Mac," he added.

"Freddie Mac was paying speaker Gingrich $1.6 million at the same time Freddie Mac was costing the people of Florida millions of dollars."

Gingrich, shedding his trademark bombast, tried to rise above the fray, as the audience looked on with a silence not seen in previous -- more raucous -- debates.

"It is the worst kind of trivial politics," Gingrich said testily, "I don't want to waste any time on it."

"Governor Romney can't tell the truth," he added, echoing criticism from Democrats about his opponent's consistency on key issues.

Gingrich said his role working for Freddie Mac had been as an "historian."

The former Georgia lawmaker released his contract with the firm just hours before the debate to try to defuse the toxic issue.

Amid the feisty back and forth, the other two Republicans still in the race, Christian conservative Rick Santorum and Texas congressman Ron Paul paled into the background, left watching as the frontrunners dueled it out.

After the aggressive opening, the rhythm settled and there were fewer fireworks than some previous encounters.

The imminent release of Romney's tax returns -- scheduled for Tuesday -- hung over proceedings like the Sword of Damocles.

Romney, thought to be worth between $190 million and $250 million, has said he pays an effective tax rate of about 15 percent, which would be in line with tax laws that Obama has denounced as lopsidedly favoring the rich.

The tax issue plays into concerns over the manner in which Romney accrued his massive wealth and his ability to relate to working-class people in an America crawling out of recession with high unemployment.

Romney has struggled to win over the Republicans conservative base who remain wary of his Mormon faith and his changing positions on key issues such as abortion and gay marriage.

But Florida is a far larger and more diverse state than the others which have so far voted, and Romney will be hoping that his campaign war chest and well-oiled machine will give him the edge.

More than 220,000 voters have already cast early ballots, a state party official told AFP. And the Rasmussen poll found Romney was leading in those votes by about 11 percentage points.

US media on Monday reported the wife of a Casino mogul will inject $5 million into a pro-Gingrich campaign group.

A Rasmussen poll found Gingrich was now running at 41 percent in Florida, with his rival at 32 percent. Just last week, Romney had a 22-point lead in the Sunshine State.

A second poll by InsiderAdvantage/Majority Opinion Research had Gingrich leading by 34.4 percent to 25.6 percent.