Mitt Romney was poised to retake the lead in the Republican White House race as Floridians went to the polls in a crucial presidential primary contest, following a caustic final campaign blitz.
Romney - a multimillionaire and former Massachusetts governor - was hoping to score a big win to restore his status as the all-but-certain Republican nominee, after several days pounding his chief rival Newt Gingrich on the campaign trail.
A heavy loss to Gingrich in South Carolina shattered Romney's air of invincibility in the contest to face off against Democratic President Barack Obama in November, and set off an avalanche of negative ads that led to a toxic campaign climate in Florida.
But Romney said on Tuesday that the defeat in South Carolina helped steel his spine and toughen his resolve.
"In South Carolina we were vastly outspent with negative ads attacking me,'' Romney told reporters outside his Tampa headquarters, also conceding that Gingrich's success in that state had been helped by his impressive debating skills.
Romney used the humbling defeat to try to show Florida voters he is made of sterner stuff than he has sometimes been given credit for.
"We responded to the attacks coming against us,'' he said. "I'll tell you, if I'm attacked, I'm not going to sit back. I'm going to fight back and fight back hard.''
Romney and former House speaker Gingrich have waged an increasingly bitter and personal battle in Florida, the largest state so far to hold balloting in the race to select the Republican presidential nominee.
After the South Carolina primary Gingrich led Romney in the polls by around five percentage points.
But a well-funded fusillade of attacks by the former governor and his supporters saw Gingrich's lead vanish in a matter of days.
So relaxed and self-assured was the usually tightly-wound Romney that he broke into song on the eve of the vote, warbling several bars of the patriotic anthem "America the Beautiful'' at a campaign event as the crowd sang along.
Polls opened at 7:00 am (0100 NZT) across the state, with results expected late on Tuesday. The winner will get all of Florida's 50 delegates, the biggest prize yet in the campaign to secure the nomination with 1,144 delegates.
Ahead of the vote, Romney led Gingrich by 13 points according to the Real Clear Politics polling average. Another local poll late on Monday showed that the gap may be as much as 20 points.
Gingrich has hit back against Romney, accusing the former venture capitalist - one of the wealthiest individuals ever to seek the presidency - of trying to spend his way to the White House.
"Money power cannot buy people power. People power depends on conservatism and we are going to take back our country,'' Gingrich said in Tampa, joined by former candidate Herman Cain, a favourite of ultra-conservative Tea Party activists.
Despite the lopsided polls, Gingrich said he expected a close race in Florida - which will be a key battleground in the general election - and has vowed to fight on in the state-by-state battle all the way to the party's convention in August.
The pugnacious Gingrich helped engineer a Republican takeover of Congress in 1994, but he became the first speaker ever reprimanded by a House ethics committee and was later pushed out of the top job in Congress.
Gingrich later created several policy think-tanks and took on work as a highly paid consultant for mortgage giant Freddie Mac, which is seen as complicit in the US financial meltdown of 2008 - steps which have led Romney to berate him as a longtime Washington insider.
One voter, 22-year-old Steve Avila, told AFP he favoured Romney because the self-made millionaire had the financial know-how to steer the country out of recession.
"I voted for Romney because he's been the most positive guy among other candidates, and I think his experience as a business man is exactly what we need now in our country,'' he said after voting in Miami.
"It's my generation that will have to pay for Obama's policies,'' said Avila, an intern at a Miami investment firm.
With seven states voting in the next four weeks, Romney's vast war chest and deep political organization could come to the fore as the candidates battle on multiple fronts.
Romney won five of those seven states in 2008, despite losing the eventual nomination to Senator John McCain.
The next vote will take place in Nevada on Saturday.
Meanwhile, former senator Rick Santorum - a religious conservative who won the first state of Iowa but whose campaign has been flagging in recent weeks - appeared to acknowledge his lack of traction in Florida and had begun campaigning in more promising states.
The other Republican in the race, congressman Ron Paul of Texas, headed to Maine on Friday.