TAMPA, Fla. (AP) _ John McCain and Mitt Romney made final-hour appeals to vote for them – and not the other guy – as Florida Republicans cast ballots Tuesday in a pivotal primary.

“I’m feeling good. Endorsements matter,” McCain said, standing alongside his chief supporter in the state, Gov. Charlie Crist, at a waterside St. Petersburg, Fla., polling station.

Across Tampa Bay, about 150 supporters greeted Romney with enthusiastic chants of “Mitt is It” at a rally at the Tampa Convention Center.

As voting began, the two Republican presidential candidates continued their weeklong volley of criticisms.

McCain argued anew that Romney’s economic record as a one-term Massachusetts governor 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.”

Romney countered with an unnamed, though obvious, slap at McCain. “One of the candidates out there running for president said that the economy is not his strong suit; well, it’s my strong suit,” Romney said. McCain has acknowledged being better versed in national security issues than economics.

Critical phone calls, negative radio ads, and bitter, personal exchanges on those two issues marked the final days of the campaign here.

Florida’s primary could solidify one of the two as the GOP front-runner; each hopes a Florida win will provide a burst of energy heading into the virtual national primary next week. The Tuesday contest offers the winner 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.

Voting began at 7 a.m. EST, but more than a million ballots had already been cast, either through the early voting that began Jan. 14 or in absentee ballots returned by mail. High voter turnout was expected statewide; a proposed state constitutional amendment on property taxes on the ballot and clear weather could boost it.

Rudy Giuliani, the former New York mayor who has lost six straight contests, is seeking a win to remain a viable candidate. But he is far behind in the polls, and a poor showing could force him to abandon his bid. Also lagging is Mike Huckabee, the former Arkansas governor who hasn’t won since the Iowa caucuses nearly a month ago and hardly competed in Florida.

In Tampa, Huckabee said he wanted to show he “was still contending” with a solid showing in Florida. 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.

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.

The contest is so tight between McCain and Romney that just about any factor could tip the balance.

More so than his rival, Romney has a get-out-the-vote effort as well as early and absentee voting programs. He’s spent considerable time and money in the state in the past year. The recent focus on the economy works in his favor; he’s been pushing his private-sector credentials and arguing that he’s the most capable to turn the country away from the brink of recession.

Conversely, McCain is backed by Florida’s top two Republican elected officials, Sen. Mel Martinez and Crist, and has endorsements from a slew of Florida newspapers. The former Vietnam prisoner of war also has universal name recognition, as well as ownership of an issue important to the large number of veterans and active military in the state – national security.

Terri Rose, 50, a Republican voting in Neptune Beach, said she cast her ballot for McCain.

“I think he would be the best on the security issues,” Rose said.

McCain is expected to do well in areas with a strong military presence – Pensacola, Jacksonville, Tampa. He’s also hoping for a strong turnout in Miami, with its heavy Cuban-American population, and Orlando, a melting pot. Romney is fighting for the southwest part of the state around Fort Myers and Sarasota; it’s much like the Midwest, where he was raised. Another likely stronghold, Palm Beach and Broward County, home to many Northeastern transplants.

Up for grabs is the corridor along Interstate 4 between Tampa and Daytona Beach, a swing area that has seen much growth and is home to roughly two-thirds of the Republican primary vote.

Associated Press Writers Glen Johnson and Anthony McCartney in Tampa, Fla., and Devlin Barrett in Broward County, Fla., contributed to this report.