ST. PETERSBURG, Fla. -- Florida's rough-and-tumble Republican primary last night rolled in favor of Arizona Sen. John McCain, who held off a challenge from former Massachusetts Gov. Mitt Romney, and strengthened his lead in the race for his party's presidential nomination.

With the win, Mr. McCain, who already had recorded victories in New Hampshire and South Carolina, picked up a whopping 57 delegates and the perception of momentum heading into next week's Super Tuesday primaries.

"It shows one thing. I'm the conservative leader who can unite the party," the Arizona senator said in a brief interview with The Associated Press. "It's a very significant boost but I think we've got a tough week ahead and a lot of states to come."

New York Mayor Rudolph W. Giuliani, who ran a distant third despite betting his entire candidacy on the state, was widely reported to be ready to endorse Mr. McCain, although he did not do so last night.

Addressing a crestfallen but supportive crowd at a rally last night in The Mirror Lake Lyceum in St. Petersburg, Mr. Romney expressed his thanks and said, "Almost, but not quite."

But he vowed to continue his quest for the nomination, saying, "Washington is fundamentally broken. And we're not going to change Washington by sending the same people back and having them sit in different chairs."

Bay Buchanan, the sister of one-time Republican presidential candidate Pat Buchanan and a volunteer for the Romney campaign, said the contest was now "just a two-man race."

"On one side we'll have the governor, a real Reagan-style conservative, and on the other side we have McCain, not just on the other side, but leading the other side. Now [the Republican Party] has a real choice."

On the Democratic ballot, New York Sen. Hillary Clinton swept to an overwhelming defeat of Illinois Sen. Barack Obama and former North Carolina Sen. John Edwards, in a race relegated by many to more of "a beauty contest" because the National Democratic Committee has said it would not count Florida's delegates. The committee took that action in response to Florida's decision to move its primary date up in violation of party rules.

The action on the Republican side, however, has been hot and heavy. All four major GOP candidates spent the past week or more crisscrossing Florida in search of support. Mr. McCain and Mr. Romney -- the candidates with the most money to spend -- also invested heavily in television advertising, mass mailings and telephone banks urging residents to get out the vote.

Mr. McCain traveled the state touting his status as a war hero, his reputation as a cost-cutter, and the totebag full of endorsements that he had picked up from such popular politicians as Connecticut Sen. Joe Lieberman and Florida Gov. Charlie Crist and Sen. Mel Martinez.

Mr. Romney countered with the buzz of conservative talk radio hosts and local campaign workers who peppered Florida's landscape with yard signs.

Mr. Giuliani, desperate for a victory, was hoping that the extra time he spent in the state -- more than any other candidate -- would be rewarded in the early voting permitted in Florida. Mr. Huckabee, a Baptist minister, relied on public appearances and the attention generated by his upset win in the Iowa caucuses.

While the Florida victory buoys Mr. McCain's supporters, Mr. Romney's fans remained steadfast last night.

Jeramie Concklin, 31, a Clearwater business owner attending Mr. Romney's rally, said he had supported him since he was governor of Massachusetts.

"His business experience is unparalleled," Mr. Concklin said. "And his accomplishments with the economy and health care [in Massachusetts] were quite remarkable.

"The second-place finisher is not out of the race," he said.

Election officials reported a heavy turnout that included well over 1 million early votes submitted by residents in the weeks before yesterday's primary. Exit polls showed that the issue of most concern to voters was the nation's economy, followed by terrorism, Iraq and immigration.

Other data showed that eight in 10 Republican primary voters were white and a little more than one in 10 were Hispanic -- about half of whom were of Cuban heritage. There were few blacks on the GOP side. In the Democratic primary, two-thirds were white, about one in five were black, and a little more than one in 10 were Hispanic.

Florida's electorate had another distinction: Both parties' electorates were older than in any other presidential contest this year. A third or more in each primary were at least 65 years old.

The candidates now advance to "Super Tuesday," when more than 20 state primaries and caucuses are held Feb. 5.

Susan MacManus, a political science professor at the University of South Florida in Tampa for 20 years, said the earlier primary date paid off, lifting Florida's national profile and generating enthusiasm -- and revenue -- statewide.

"The [state] party worked hard to tell the national party, 'Hey, you have to pay attention to us,'" she said. "The fact that [the races have] been neck-and-neck for over a week has created excitement. And it's equally enthusiastic on both sides of the aisle. [Florida voters] have the best seat in the country, right now, as far as politics is concerned."

But Ms. MacManus said the close race had a downside, as it turned ugly in the last week with Mr. Romney and Mr. McCain trading accusations and each labeling the other as "liberal."

"It's been very disappointing, especially after the Boca Raton debate," Ms. MacManus said. "The tone had been so respectful and focused on the issues, but it disintegrated into mudslinging."

Florida's Republican-controlled Legislature moved the date of the state's primary from March to January in hopes of giving voters here a greater say in which candidates would be running in November. The earlier date, however, flew in the face of party commitments to Iowa and New Hampshire, which then moved their election dates up further.

The Democratic National Committee, which last year stripped Michigan of its delegates for a similar move, said Florida's 185 delegates to the national convention would not be seated. Democratic candidates subsequently signed pledges saying they would not campaign in Florida. Fund-raising, however, was still allowed.

Mrs. Clinton attended fund-raisers Sunday in the state and returned last night to a rally in south Florida after the polls closed, vowing to work to seat the delegates of Michigan and Florida.

Supporters of Mr. Obama and Mr. Edwards have cried foul, saying Mrs. Clinton is going back on her pledge. They also pointed out that, because they were not able to campaign in Florida, their messages did not get out and Mrs. Clinton won simply on the basis of her name recognition.

Ms. MacManus, however, said such is the hard-fought world of politics.

"[The Democratic Party] realizes how important Florida Democrats are," she said. "Florida has two things that politicians simply can't live without: delegates and money."

Democratic voters turned out in strong numbers yesterday even though it's possible that their choice for presidential candidate might be ignored by the national party.

St. Petersburg resident Diane Maher, 64, emerged from the polling place in the Peterborough Apartments, where she voted for Mrs. Clinton, saying she hoped that the Democratic Party would seat her state's delegates. Regardless, she said, she was determined to vote.

"There were other things on the ballot," she said, referring to, among other things, a controversial tax amendment to Florida's Constitution. "And I wanted to vote for Hillary. I believe she's the best choice."

M.A. Musselman, 61, a nurse from St. Petersburg, said she voted for Mr. Obama because "he brings to the people of the United States something that we have been struggling for more than 100 years. He has shown his insight and integrity, his intelligence and his grace."