Two days before Floridians vote, polls showed Romney pulling clear in the Sunshine State after his campaign team went on the offensive, unleashing blistering ads that painted Gingrich as unethical and not fit for office.
Romney, a former Massachusetts governor and millionaire venture capitalist considered the party establishment's favorite, was leading Gingrich by 15 points in Florida, 42 percent to 27 percent, according to an NBC/Marist poll.
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A new Miami Herald poll showed Gingrich behind by 11 points.
Hoping to finish off his rival, Romney renewed his assault before a crowd of several hundred supporters in an affluent Naples shopping center.
"The reason that speaker Gingrich has been having a hard time in Florida is that people of Florida have watched the debates, have listened to the speaker, have listened to the other candidates and have said, 'you know what, Mitt Romney's the guy we're going to support,'" he said.
Despite the polls, Gingrich said he expected a close race in Florida, which will be a key battleground in the November general election, pitting President Barack Obama, a Democrat, against the eventual Republican nominee.
After attending church services in this community north of Tampa, a fiery Gingrich insisted "the election will be substantially closer than the two polls" suggest.
With time running out for Gingrich to claw back lost ground in Florida, he turned to the national fight.
"Romney has got a very big challenge in trying to get a majority at the convention," he said, predicting a "wild and woolly" campaign ahead.
"We will go all the way to the convention, and I believe the Republican Party will not nominate a pro-abortion, pro-gun control, pro-tax increase moderate from Massachusetts."
Romney, who has switched positions on abortion and is now pro-life, has had to fend off lingering doubts over his conservative credentials dating back to his time as governor of liberal Massachusetts.
Gingrich, 68, shocked the party establishment when he thumped Romney, 64, in South Carolina earlier this month, but his support has been sinking fast in Florida and his opponent now appears to be the one with all the momentum.
A crucial role is being played by Christian conservative Rick Santorum, who won the first state of Iowa but whose campaign has been flagging since.
Gingrich insisted Sunday that the only reason he trailed the more moderate Romney was because of his rival's "relentlessly negative campaign" and because Santorum, by staying in the race, was splitting the conservative vote.
"The fact is, when you combine the Santorum vote and the Gingrich vote... the conservative combined would clearly beat Romney," he told ABC talkshow "This Week."
Gingrich's campaign got a weekend boost when he received an endorsement from former rival Herman Cain. Former Alaska governor Sarah Palin, a favorite of the ultra-conservative Tea Party wing of the Republican Party, also weighed in, telling supporters to "Vote for Newt" in Florida.
"We need somebody who is engaged in sudden and relentless reform and is not afraid to shake up the establishment," Palin told Fox News.
"They want to crucify him because he's tapped into that average everyday American Tea Party, grass-roots movement that has said enough is enough of the establishment."
Gingrich has sought to rally support by showcasing his conservative credentials, arguing that only a true conservative like himself has a chance to beat Obama.
"We nominated a moderate in '96 and we lost," he said Saturday in Orlando. "We nominated a moderate in 2008 and we lost. Only a solid conservative can debate Barack Obama and win."
But seizing on his rival's precarious position, Romney's ad used television news footage from 1997 when Gingrich became the first speaker ever reprimanded by the House of Representatives.
Gingrich was accused of dozens of violations, including a claim of tax-exempt status for his college course.
Romney has touted his business experience as being better suited to the tasks of creating jobs and bringing strong growth to a lackluster US economy -- and has poured scorn on Gingrich's background as a Washington lawmaker.
But Romney's own business record has attracted challenges, and he has been on the defensive over his immense wealth and low tax rates, revealed in the recent release of his 2010 tax return.
