MIAMI -- Florida Sen. Mel Martinez endorsed John McCain on Friday, a move likely to give the Republican presidential candidate a crucial boost with the state's Cuban-Americans just days before the primary.

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Meanwhile, Republican presidential candidates begin their last weekend of campaigning.

On Saturday, Mitt Romney will be in St. Petersburg, Lakeland and Lutz. Rudy Giuliani will be in Sarasota and Orlando. And John McCain will be in North Fort Myers and Sun City.

In his endorsement of McCain, Martinez praised his chosen candidate.

"I understand that he is ready on Day One to lead this nation, and I would trust the future and the security of this nation to this man," Martinez said in his introduction of McCain at the Latin Builders Association.

He added: "I would not endorse someone that I didn't have total confidence is going to be (Fidel) Castro's worst nightmare," repeating the sentence in Spanish.

The decision is a blow to Rudy Giuliani, the former New York mayor in a close fight with McCain for support of voters in the Cuban-American community -- and to keep his candidacy alive. It also is a setback for Mitt Romney, the former Massachusetts governor in a close race against McCain.

Martinez, a Republican who was born in Cuba, emigrated to the United States as a teenager and is popular in Miami's Little Havana neighborhood. It remains to be seen whether Martinez' endorsement will translate into votes given that a significant percentage of voters already have cast absentee and early ballots -- many before Giuliani's drop and McCain's ascent in the state.

But the backing of Florida's junior senator may help McCain, nonetheless, in huge population centers of Miami, where many Cuban-Americans live, and Martinez' hometown of Orlando.

Florida's primary is Tuesday, and polls show McCain in a close race with Romney while Giuliani trails in his must-win state.

Martinez and the four-term Arizona senator are longtime friends who worked closely together on among other issues, a bill that would have created an eventual path to citizenship for millions of illegal immigrants in the country.

As recently as Thursday night, Martinez indicated he would remain neutral in the race despite his friendship with McCain. He decided to endorse the Arizona senator Friday morning after conversations with several McCain supporters and his wife, officials say. The move surprised even his closest advisers.

He plans to spend the three days before the primary campaigning for McCain.

Martinez is a first-term senator who served as President Bush's secretary of Housing and Urban Development from 2001 until 2003. Last October, he stepped down as the general chairman of the Republican National Committee after serving only 10 months.

Martinez, who is up for re-election in 2010, said he was relinquishing the job to spend more time focusing on his Florida constituents. He also said the RNC had achieved the objective he set when he assumed the job. A Quinnipiac University poll in October showed that Martinez only had 35 percent approval rating, and only 48 percent among Republicans.

His role with the national Republican Party likely hurt him with Democrats and independents, while his push for an immigration bill probably undercut his support among some Republicans.

Three members of Miami-area members of Congress -- Reps. Lincoln Diaz-Balart, Mario Diaz-Balart and Ileana Ros-Lehtinen -- previously have given McCain their support.