By Tim Gaynor

MIAMI (Reuters) - John McCain scored a hard-fought win in Florida's presidential primary on Tuesday, seizing the front-runner's role in a heated Republican race and possibly ending one-time favorite Rudy Giuliani's White House bid.

McCain, an Arizona senator, defeated former Massachusetts Gov. Mitt Romney in a tight Florida battle that gives him critical momentum heading into critical February 5 "Super Tuesday" voting in 21 states with Republican contests.

The result could mean the end for Giuliani, a former New York mayor who staked his campaign on a strong showing in Florida but was battling former Arkansas Gov. Mike Huckabee for a distant third-place finish after leading national polls for much of the year.

Giuliani reportedly was pondering dropping out and endorsing McCain as early as Wednesday, and talked about his campaign in the past tense during a speech to supporters in Orlando, Florida.

"We ran a campaign that was uplifting," Giuliani said. "You don't always win, but you can always try to do it right."

McCain's win put him at the front of the pack in a seesawing Republican race to pick the party's candidate in November's presidential election. He picks up all of Florida's 57 delegates to the national nominating convention.

New York Sen. Hillary Clinton easily won a Florida Democratic race that featured no active campaigning because of a dispute between the national and state parties. The national party stripped the state of its delegates to the national convention and Democratic candidates pledged to stay away.

ENDORSEMENTS

Clinton, who lost to rival Barack Obama in a landslide in South Carolina on Saturday, visited the state after polls closed in a bid to claim at least a symbolic victory.

"Thank you Florida. I could not come here to ask in person for your votes but I am here to thank you for your votes," she told supporters in Davie, Florida.

McCain and Romney had dominated the headlines in Florida with a heated battle over who was best prepared to rescue a struggling economy and lead a country at war in Iraq and Afghanistan.

"I think it's time for the politicians to leave Washington and for the citizens to take over," Romney told supporters in St. Petersburg, Florida. "At a time like this, America needs a president in the White House who has actually had a job in the real economy."

McCain had gained in polls in recent days since his endorsements by Florida Gov. Charlie Crist and U.S. Sen. Mel Martinez of Florida.

McCain and Romney had split the last four of the state-by-state nominating contests. McCain won in South Carolina and New Hampshire and Romney carried Michigan and Nevada, the latter a state scarcely contested by other Republicans. Huckabee won the kick-off contest in Iowa.

Huckabee also said he planned to go on to compete in the February 5 contests, which include several Southern states like his home state of Arkansas, Alabama, Oklahoma, Tennessee and Georgia.

"We're a long way from quittin'," he said on Fox News Channel.

(Writing by John Whitesides; additional reporting by Jim Loney and Jason Szep in Florida and Jeff Mason in Kansas; editing by Patricia Zengerle)

(For more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http:/blogs.reuters.com/trail08/)