Republicans in Florida are deciding whether a confident Mitt Romney or a defiant Newt Gingrich will win the state's 50 delegates, the biggest prize of the race for the White House nomination so far.
Mr Romney is heavily favoured in the winner-takes-all primary, the final and possibly pivotal contest in a high-stakes month in which he has claimed one win and two second-place finishes.
But Mr Gingrich has dismissed suggestions that he could be hobbled by a significant loss in Florida, telling reporters the race would not be decided until June or July - "unless Romney drops out earlier". He complained that Mr Romney had affected his campaign with heavy spending on "ads that are dishonest".
Mr Romney said he had been forced to defend himself after a stunning loss to Mr Gingrich in the January 21 South Carolina primary. He attributed that defeat to negative adverts aired on Mr Gingrich's behalf.
"I needed to make sure that instead of being outgunned in terms of attacks, that I responded aggressively, and hopefully that will have served me well here," Mr Romney told reporters.
Mr Romney has been the front-runner for much of the race and appeared likely to recapture that role after the Florida vote. He and his allies have poured more than 14 million US dollars (£8.8 million) into Florida television advertising primarily to attack Mr Gingrich, who has struggled to compete with his rival's fundraising ability, staffing and network of high-profile supporters.
Mr Gingrich and his team spent roughly 3 million US dollars (£1.9 million) on Florida advertising.
The last Florida polls close at 8pm local time. Republican officials in the state are anticipating a big turnout, more than two million voters, up from a record 1.9 million in the Republican primary in 2008.
The other two candidates in the race are not contesting Florida.
Rick Santorum and Ron Paul have ceded Florida's primary to Mr Romney and Mr Gingrich in favour of smaller, less-expensive contests.