Floridians are set to vote in the state's Republican primary as rivals Mitt Romney and Newt Gingrich look to capitalise on earlier wins.
Romney has surged ahead in the Florida polls, ahead of Gingrich by 15% in the most recent survey.
The other hopefuls, Ron Paul and Rick Santorum, have abandoned campaigning in Florida, making it in effect a race between two candidates.
Latinos and retired voters make up key voting blocs in the state.
Both Romney and Gingrich are looking for increased momentum after early primaries split wins among three different candidates.
Polling stations will be open from 0700 to 1900 (1200-0000 GMT). Early voting has been allowed for the past 10 days, and it is estimated that 600,000 people have already voted.
Republicans are vying to oust Barack Obama from the White House in November.
Primaries and caucuses will be held in every US state to pick a Republican candidate until the eventual winner is crowned at the party's convention in August.
Gingrich scored a resounding victory in South Carolina's primary earlier this month, but his momentum has since been stymied in the face of a Romney fightback.
Both candidates crisscrossed Florida on Monday in a final hunt for votes.
"My goal would be by the end of that first day, about the time that President Obama arrives back in Chicago, that we will have dismantled about 40% of his government," Gingrich told voters in Jacksonville, Florida as part of his stump speech.
Romney finished his day of campaigning at The Villages, a retirement community that has become a popular political stop.
The former Massachusetts governor had attacked Gingrich throughout last week but spent most of the rally focusing on Obama.
Reagan's son campaigns
Several surveys released on Sunday gave Romney a lead of between five and 16 points.
But Gingrich vowed to continue his campaign in the Sunshine State and beyond, attacking Romney's conservative credentials.
"On big philosophical issues, he is for all practical purposes a liberal and I am a conservative and that's what this fight is going to be about all the way to the convention," he told CBS News on Monday.
At least 1,114 delegates are needed to win the Republican nomination at the August convention. In Florida, there are 50 delegates are at stake.
"This race is just getting started," Martin Baker, Gingrich's national political director, told reporters, saying there was "a long way to go" before the nomination was decided.
The former House speaker also campaigned with former President Ronald Reagan's son Michael on Monday, hoping to strengthen his campaign's ties to the revered Republican leader's legacy.
States in which the electorate is relatively evenly split between Republicans and Democrats, making them targets for aggressive campaigning by both sides. In recent elections, the most important swing states were Florida, Ohio and Pennsylvania. Those have a high number of electoral votes, making them prime battlegrounds during the election. The list of swing states changes with their demographics. In the 2008 election, for instance, historically Republican Virginia and North Carolina voted for Barack Obama, anticipating their status as hard-fought swing states in 2012. Others that were close in previous elections, like Iowa and New Mexico, appear to be solidly Democratic.
Former Pennsylvania Senator Rick Santorum left Florida over the weekend to be at the hospital bedside of his three-year-old daughter, who has a rare genetic condition.
Rather than return to Florida, he will campaign in Colorado, Minnesota, Missouri and Nevada over the next two days.
Libertarian Texas congressman Ron Paul, meanwhile, is already focusing on Nevada.
Gingrich picked up the endorsement on Saturday night of former rival Herman Cain, who dropped out of the Republican race in December.
The former House speaker is portraying himself as the only viable conservative in the race.
Romney has struggled to dispel misgivings among some Republicans about his political record as governor of the liberal state of Massachusetts and his Mormon faith.
In the past, Romney has adopted a pro-choice stance on abortion, switched positions on gay rights and pushed through a healthcare reform in Massachusetts that was the template for President Obama's overhaul of the system nationally.
The next contest after Florida will be Nevada on Saturday.
