Romney and Giuliani in 2007 (Joe Raedle/Getty Images)
In an interview on "Fox & Friends," the former New York City mayor said Romney has proven he's the "most effective Republican" in the race.
"Mitt has won fair and square," Giuliani said. "He's taken on everybody and won an incredible number of primaries, and he's got the resume and background for the job."
Giuliani's backing comes after years of open animosity between the former political rivals, who ran against each other during the 2008 Republican presidential primary.
While the former mayor had not formally endorsed a candidate this cycle until now, Giuliani wasn't shy about criticizing Romney's campaign in the past and frequently described Newt Gingrich as a "stronger" candidate.
On Fox News, Giuliani acknowledged he would be supporting Gingrich had the former House speaker run a better campaign. He also admitted he and Romney had not always been close political allies. But Giuliani said he decided to formally throw his backing to Romney after having "a long talk" over breakfast last week.
"We've talked a number of times, but this was a really good, long talk," Giuliani said, explaining that he had quizzed Romney on foreign policy and economic issues.
The ex-mayor said Romney has "an understanding of the economy that's far deeper than the president and far deeper than most people" and praised his "strength and toughness" on foreign policy.
Giuliani is the latest Republican holdout to back Romney's nomination. Last week, the ex-governor won the support of three other high-profile Republicans who had been neutral in the race: House Speaker John Boehner, Senate Minority Leader Mitch McConnell and Indiana Gov. Mitch Daniels.
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