The Arizona senator is locked in a tight race with Mitt Romney, with Rudy Giuliani and Mike Huckabee trailing. All sought Crist's endorsement.

McCain was appearing with Crist at the Pinellas County GOP Lincoln Day Dinner later Saturday, and the endorsement was to be announced there, according to Republican officials who spoke on the condition of anonymity.

Crist's move comes a day after Florida Sen. Mel Martinez gave his nod to McCain and promised to vigorously campaign on his behalf in the final days of the campaign. Officials said Martinez had been lobbying Crist over the past 24 hours to side with McCain.

It's unclear what effect the two endorsements will have on McCain's candidacy.

At the very least, the nods of Florida's two top Republican elected officials could serve to validate McCain's candidacy with the GOP establishment and counteract the fears among some that he would not be a loyal Republican while in the Oval Office.

Crist, a popular first-term governor, had suggested he would stay out of the multi-candidate GOP primary, and played coy about his preferences for a year. He met with all the serious contenders, and appeared with some at events.

McCain campaigned for Crist as he sought the governor's office, well before he officially launched his White House bid.

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