(AP Photo/ Joseph Kaczmarek)

POMPANO BEACH, Fla. - Newt Gingrich slammed Republican rival Mitt Romney on Sunday for the steady stream of attacks he likened to "carpet-bombing," trying to cut into the resurgent front-runner's lead in Florida in the dwindling hours before Tuesday's pivotal presidential primary.

And despite surging ahead in polls, Romney wasn't letting up, relentlessly casting Gingrich as an influence peddler with a "record of failed leadership."

In what has become a wildly unpredictable race, the momentum has swung back to Romney, staggered last weekend by Gingrich's big upset victory in South Carolina, a major setback in the state-by-state race for enough delegates to lock up the right to challenge President Barack Obama in November.

An NBC News/Marist poll published Sunday showed Romney with support from 42 per cent of likely Florida primary voters, compared with 27 per cent for Gingrich.

Romney's campaign, which has a huge financial advantage, has dogged Gingrich at his own campaign stops, sending surrogates to remind reporters of Gingrich's ethics probe while he was speaker of the House of Representatives in the 1990s and other episodes in his career aimed at sowing doubt about his judgment.

Gingrich reacted defensively, accusing the former Massachusetts governor and a political committee that supports him of lying, and the Republican party's establishment of allowing it.

"I don't know how you debate a person with civility if they're prepared to say things that are just plain factually false," Gingrich said during appearances on Sunday talk shows. "I think the Republican establishment believes it's OK to say and do virtually anything to stop a genuine insurgency from winning because they are very afraid of losing control of the old order."

Outside an evangelical Christian church in Lutz, Gingrich said he was the more loyal conservative on key social issues than Romney, attacking him for his shifting positions on key social issues since he served as governor of the Democratic state of Massachusetts.

Romney continued to paint Gingrich as part of the very Washington establishment he condemns and someone who had a role in the nation's economic problems.

Gingrich's consulting firm was paid more than $1.5 million by a federally-backed mortgage company despised by conservatives after he left Congress in 1999.

The intense effort by Romney to slow Gingrich is comparable to his strategy against Gingrich in the closing month before Iowa's leadoff caucuses Jan. 3. Gingrich led in Iowa polls, lifted by what were hailed as strong performances in televised debates, only to drop in the face of withering attacks by Romney, aided immensely by ads sponsored by a "super" political action committee run by former Romney aides.

But Romney aides say they made the mistake of assuming Gingrich could not rise again as he did in South Carolina. Romney appears determined not to let that happen again.

"His record is one of failed leadership," Romney told more than 700 people at a rally in Pompano Beach Sunday evening. "We don't need someone who can speak well perhaps, or can say things we agree with, but does not have the experience of being an effective leader."

Gingrich, in appearances on Sunday news programs, returned to complaining about Romney's tactics. "It's only when he can mass money to focus on carpet-bombing with negative ads that he gains any traction at all," he said.