MIAMI — Senator McCain and Mitt Romney clashed in a hard-fought Florida primary today, seeking campaign momentum before the race for the Republican presidential nomination turns into a nationwide delegate struggle on February 5.

Mayor Giuliani and Michael Huckabee also had spots on the ballot, struggling for survival in a race that threatened to leave them behind — or out.

The winner stood to gain all 57 national convention delegates at stake, the biggest prize so far in an early round of primaries and caucuses.

The Democratic primary was controversial by its very existence, held earlier in the year than national party officials had wanted. That made it a nonbinding popularity contest with no delegates awarded on the basis of the outcome.

But even that became grist for disagreement in the escalating battle between Senators Clinton and Obama.

All the Democratic candidates agreed in advance not to campaign in the state. Mrs. Clinton, who was routed in the South Carolina primary last weekend, repeatedly sought to draw attention to an event she expected to win. Without success, she challenged her rivals to agree to heed the results of the Florida contest — and of the January 15 primary she won in Michigan, which was also stripped of its delegate for leapfrogging ahead — when it comes time to seat delegates at next summer’s Democratic National Convention.

“Those decisions will be made after the nomination, not before,” Mr. Obama told reporters today on a plane from Washington to Kansas. “Obviously, I care a lot about the people in Michigan and a lot about the people in Florida. And I want their votes in the general election. We’ll be actively campaigning for them.”

Mr. Romney began the evening with 59 Republican delegates, to 36 for Mr. McCain, and 40 for Mr. Huckabee. Mr. Giuliani had one.

No matter the winner, there was no time to rest. There are 23 contests on the ballot on February 5, with 1,023 delegates at stake.

Mr. McCain milked his endorsement from Florida’s highest-ranking Republican today, appearing outside a polling station alongside Governor Crist as voting began in the primary.

“It’s a true honor to stand with this American hero, a great statesman, a tremendous United States senator, and a great leader for this country,” Mr. Crist said during a syrupy made-for-TV appearance in St. Petersburg. Mentioning the endorsement, Mr. Crist added: “I hope it helps the guy.”

Messrs. McCain and Romney clashed early and often, in personal appearances and paid television advertising, in a bruising week of campaigning in Florida.

The former Massachusetts governor said his career as a private businessman made him perfectly suited to sit in the Oval Office with a recession looming. Mr. McCain argued he knew his economics well enough, and that his career in the military and in Congress made him the man to steer the country in an age of terrorism.

By the campaign’s final hours, the two men hurled insults at one another, each saying the other hoped to travel a liberal road to the presidential nomination in a party of conservatives.

Mr. Romney attacked Mr. McCain for his signature legislation to reduce the role of money in politics, for his position on immigration, and for his support of an energy bill that he said would have driven up consumer costs.

“If you ask people, ‘look at the three things Senator McCain has done as a senator,’ if you want that kind of a liberal Democrat course as president, then you can vote for him,” Mr. Romney told campaign workers. “But those three pieces of legislation, those aren’t conservative, those aren’t Republican, those are not the kind of leadership that we need as we go forward.”

Mr. McCain had a ready reply. “On every one of the issues he has attacked us on, Mitt Romney was for it before he was against it,” 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.”

That wasn’t all, either.

Mr. McCain aired radio commercials criticizing Mr. Romney, and his campaign Web site has an ad superimposing Mr. Romney’s face on the image of a windsurfing Senator John Kerry, the 2004 Democratic presidential nominee.

The Romney campaign also reported numerous negative phone calls, accusing him incorrectly of supporting taxpayer-funded abortions, opposing President Bush’s tax cuts, and favoring direct talks with the Cuban leader, Fidel Castro.

The McCain campaign said it was not responsible for the calls.