Once the front-runner himself, Giuliani decided to abandon the race after a dismal performance in Tuesday's Florida primary, a contest on which he had bet his political fortune. Instead, McCain won and Giuliani came in a distant third.
Giuliani recalled he had said in an earlier debate that McCain would be his choice for president if he were not running himself.
"If I'd endorsed anyone else, you would say I was flip-flopping," he said, mentioning an oft-repeated criticism of McCain's chief rival, Mitt Romney.
McCain, standing at Giuliani's side, acknowledged his former rival as "my strong right arm and my partner."
"This man is a national hero and I'm honored by his friendship," he said.
The endorsement joined two Republicans who had campaigned on similar themes that highlighted their national security credentials - McCain's status as a Vietnam POW, war hero and a Senate voice on defense matters, and Giuliani as a stalwart New York mayor during the Sept. 11 terrorist attacks.
Giuliani announced his exit from the race and backing of McCain at the Ronald Reagan Library, site of Wednesday night's debate involving the remaining GOP candidates.
Tuesday's result was a remarkable collapse for Giuliani. Last year, he occupied the top of national polls and seemed destined to turn conventional wisdom on end by running as a moderate Republican who supported abortion rights, gay rights and gun control.
The results seriously decimated Giuliani's unconventional strategy, which relied heavily on Florida to launch him into the coast-to-coast Feb. 5 nominating contests.
But Florida proved to be less than hospitable. His poll numbers dropped and key endorsements went to McCain.
Surveys of voters leaving polling places Tuesday showed that Giuliani was getting backing from some Hispanics, abortion rights supporters and people worried about terrorism, but was not dominating in any area.
McCain, addressing his own supporters moments later in Miami, gave Giuliani a warm rhetorical embrace, a possible prologue to accepting Giuliani's expected support.
"I want to thank my dear friend, my dear friend Rudy Giuliani, who invested his heart and soul in this primary and who conducted himself with all the qualities of the exceptional American leader he truly is," McCain said. "Thank you, Rudy, for all you have added to this race and for being an inspiration to me and millions of Americans."
Giuliani hung his bid for the Republican presidential nomination on his leadership. His stalwart performance as New York mayor in the tense days after the Sept. 11, 2001, terrorist attacks on New York and Washington earned him national magazine covers, international accolades and widespread praise.
Steadfast in a crisis, as a candidate Giuliani was a bundle of contradictions, so much so that he liked to joke that even he didn't always agree with himself.
A moderate-to-liberal New Yorke, Giuliani became a Republican mayor of an overwhelmingly Democratic city. Campaigning for national office, he claimed to have created the most conservative government in the most liberal city in America.
After earning a reputation as a tough-talking, even abusive executive, Giuliani the presidential candidate was mostly mild-mannered in debates, even as those around him got meaner.
Giuliani, 63, first gained prominence as a crime-busting federal prosecutor in New York City. Jailing mob bosses, Wall Street executives and corrupt politicians helped propel his next career as a politician, but it wasn't an immediate success. He lost the first time he ran for mayor in 1989 before winning in 1993.
As mayor, he fostered a take-charge image by rushing to fires and crime scenes to brief the press, but some critics felt he was more concerned about taking credit from others for what became a historic decline in the city's crime rate during his tenure.
A bout with prostate cancer and the very public breakup of his marriage with second wife Donna Hanover - she first learned he was filing for divorce when he made the announcement at a televised news conference - forced Giuliani to withdraw from a race for the U.S. Senate against Democrat Hillary Rodham Clinton in 2000. The messy divorce was revisited in awkward detail once he re-entered politics.
With no working strategy in his presidential campaign, no primary victories and dwindling resources, the mayor's third-place finish in Florida spelled the end of his run, even if his crestfallen supporters couldn't believe it.
"They'll be sorry!" a woman with a New York accent called out to the mayor as he spoke. "You sound like my mother," Giuliani joked.
