It would also provide a burst of support in the run-up to February 5, called Super Tuesday, when 24 states hold nominating contests with more than 1,000 delegates at stake.
McCain, the Arizona senator, held a slim lead in the latest opinion polls, with 35 per cent support.
Mitt Romney, the governor of Massachusetts, stood at 31 per cent.
The battle for third saw Rudy Giuliani, a former mayor of New York, neck and neck with Mike Huckabee, Arkansas' former governor, with 13 per cent each.
Giuliani is seeking a win to remain a viable candidate, but a poor showing could force him to abandon his bid after lagging behind in the six previous votes.
Huckabee, an ordained Baptist minister, has not had a win since the Iowa caucuses nearly a month ago.
Acrimony has replaced the recent civility of the Republican race. On Monday, Romney and McCain accused each other of being liberals.
Romney criticised the Arizona senator for "liberal answers" to the country's problems, such as McCain's legislation curbing money in politics, his stance on illegal immigrants and his backing of an energy bill that Romney said would raise consumer costs.
"And I just don't think those liberal answers are what America is looking for, not for the Republican Party or for any party, for that matter," Romney said in Fort Myers, Florida.
McCain accused Romney of "wholesale deception of voters" and of flip-flopping on the issues.
He said: "The truth is, Mitt Romney was a liberal governor of Massachusetts who raised taxes, imposed with Ted Kennedy a big government mandate health care plan that is now a quarter of a billion dollars in the red, and managed his state's economy incompetently, leaving Massachusetts with less job growth than 46 other states."
Hillary Clinton was the clear favourite in the state's Democratic race, but the primary offers her only a symbolic boost as Florida has been stripped of delegates for violated party rules and changing the date of its vote.
Barack Obama, Clinton's main rival, meanwhile gained a boost on the national stage heading into Super Tuesday after gaining the endorsement of Edward Kennedy, a US senator and youngest brother of late president John F Kennedy.
Kennedy, an influential Democratic and liberal voice, on Monday described Obama as an inspirational uniter.
"He is tough-minded but he also has an uncommon capacity to appeal to 'the better angels of our nature'," Kennedy said, quoting from President Abraham Lincoln's first inaugural address in 1861.
The Kennedy clan is split, however, on who to support in the 2008 race.
Caroline Kennedy, the daughter of the late president, sided with Edward Kennedy, while Kathleen Kennedy Townsend, another of his nieces, said on Sunday that she would support Clinton.
With split decisions in the Democratic contests so far - Obama won Iowa and South Carolina, Clinton won New Hampshire and Nevada - there is increased speculation that the nominating race will extend beyond February 5.