"Deciding who to endorse isn't difficult," Giuliani said in a joint news conference with his friend, Sen. John McCain, R-Ariz. in California. "John McCain is the most qualified candidate ... ."

Giuliani said he thought the next president should be someone who could be trusted in a time of crisis, have a clear vision, and the will and perseverance to accomplish goals.

"Obviously, I thought I was that person. The voters made a different choice," Giuliani said. "I am officially announcing my withdrawal as a candidate for the presidency of the United States."

McCain said he was honored to receive Giuliani's endorsement. He said Giuliani would be "my strong right arm" for the rest of his campaign for the White House.

Giuliani eschewed heavy campaigning in Iowa and other states, putting all his eggs in Florida -- spending lots of time and money in the Sunshine state. He finished a distant third behind McCain and former Massachusetts Gov. Mitt Romney in Tuesday's Republican primary.