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.

Two Republican officials disclosed the upcoming endorsement on the condition of anonymity to avoid pre-empting the announcement.

As recently as Thursday night, Martinez indicated he would remain neutral in the race, though noted that he was close friends with McCain.

Martinez, who was born in Cuba, emigrated to the United States as a teenager.

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

Martinez and McCain are longtime friends from the Senate, and worked closely together on a bill that would have created an eventual path to citizenship for millions of illegal immigrants in the country.

The two will appear together at the Latin Builder's Association meeting, where the endorsement will be announced. Romney spoke to the group earlier, and Giuliani was finishing up his speech when the news broke. He left without taking questions from reporters.

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 in January.

Martinez had shared the chairmanship with Mike Duncan, a longtime RNC official who has been responsible for the party's day-to-day operations.

Martinez is the fourth prominent Cuban-American lawmaker to back McCain. Three members of Miami's congressional delegation—Reps. Lincoln Diaz-Balart, Mario Diaz-Balart and Ileana Ros-Lehtinen—previously gave McCain their support.

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