A potentially record number of voters headed to the polls Tuesday in Florida's presidential primary, as GOP front-runners Mitt Romney and John McCain both predicted victory ahead of the close of polls at 8 p.m. ET.

With the weather clear and and a proposed state constitutional amendment on property taxes also on the ballot, about 1 million Republicans were expected to vote Tuesday, Florida officials reported. Add to that about 500,000 who cast their selections through absentee ballots and early voting and that makes up nearly 40 percent of the 3.8 million registered Republicans in The Sunshine State.

About 1.4 million Democrats were also expected to vote Tuesday, even though the party's national committee stripped the state of its delegates for holding an early primary. The Democratic candidates agreed in advance not to campaign in Florida, but Hillary Clinton has recently drawn attention to Florida, a state she expects to win, saying the state's votes should count.

Click here to see a photo essay of primary voting day in Florida.

Turnout was high as the bitter back-and-forth between Romney and McCain turned to the grand finale, with each trying to expose the other's weaknesses in a play to get a last-minute nudge in the state's airtight GOP primary.

Florida's primary is the last major opportunity for the candidates to snag momentum before Super Tuesday Feb. 5.

McCain shot to the top of Florida polls after winning South Carolina 10 days ago. He has since been backed by Florida's top two Republican elected officials, Sen. Mel Martinez and Gov. Charlie Crist, and he has endorsements from a slew of Florida newspapers.

McCain campaigned with Crist Tuesday, standing alongside the late-game endorser saying "I'm feeling good. Endorsements matter."

FOX News exit polls seemed to indicate that endorsements do matter. McCain was leading among seniors, Hispanics and those who said Crist's endorsement was important. Romney was leading among evangelicals and individuals who put illegal immigration at the top of their list of most important issues.

Romney was behind McCain in exit polling among people who said the economy was the most important issue, Romney's strong suit, while the two were tied among veterans, a group pursued by McCain, a Vietnam veteran.

Trading barbs with renewed urgency Tuesday, McCain went after Romney for being weak on national security, while Romney went after McCain for being weak on economic security.

"One candidate out there running for president said the economy is not his strong suit," Romney told a crowd of 150 in Tampa Tuesday morning. "Well it is my strong suit.

Romney has put nine times more ads on TV in Florida than McCain. He's sharpened attacks on his top rival for sponsoring legislation with Democrats, voting against the Bush tax cuts and allegedly being a symbol of Washington insiders.

McCain, speaking in St. Petersburg, argued anew that Romney's economic record as a one-term governor of Massachusetts wasn't as strong as his rival was making it seem.

And, he said: "The real key, I think, here in Florida is who can keep America safe. Who is it that has got the experience and background and knowledge to take on the challenge of radical Islamic extremism? Governor Romney has no experience there."

The race between the leading candidates has been muddied by critical phone calls, negative ads and bitter personal exchanges. Each has resorted to describing the other as "liberal."

McCain's campaign released a Web ad Monday called "A Tale of Two Mitts," accusing Romney of flip-flopping on abortion and gun control and questioning his loyalty to Republican ideals.

Attempting to benefit from the bickering is former New York Mayor Rudy Giuliani, who like Democrat John Edwards in the South Carolina primary held Saturday, said voters will turn away from the bickering of the front-runners.

Edwards placed third-- but Giuliani says he'll win and hopes the state's early voting will work to his advantage.

Giuliani staked his campaign on Florida, but as the one-time front-runner bypassed other high-profile, early voting states, polls showed his popularity plummeting: He now sits third going into the Florida primary.

"We are going to win Florida and not going to think about anything else but to win," Giuliani told FOX News Tuesday. "We think we have a great margin in the early vote."

The Tuesday winner-take-all contest hands the winner all of the state's 57 delegates to GOP nominating convention and serves as a gateway to the 20-plus states with nominating contests on Feb. 5.

Giuliani spent the morning in the Miami area, then visited his Broward County campaign headquarters, where he reminisced about his first run for mayor with a local Florida volunteer who was also part of that 1989 effort. Giuliani picked up a cell phone and made a call to a voter who said she would vote for him. "I remember when I first ran for mayor the phone banks were actually phones, real phones," said Giuliani.

Also lagging is former Arkansas Gov. Mike Huckabee, who hasn't won since the Iowa caucuses nearly a month ago.

In Tampa Tuesday morning, Huckabee visited a local golf course and talked up his chances in the state, MyFox Tampa Bay reported.

"I've worked hard here, we've been here every day," he said. "We feel like we're gonna have a good day here."

But he also made clear he was looking forward to next week when a slew of Southern states vote. "Those are the states where we feel like becomes a very big stand for us," he said.

Huckabee plans to watch the Florida returns from Missouri, another Feb. 5 state.

FOX News' Carl Cameron, Jake Gibson and Shushannah Walshe and The Associated Press contributed to this report.