"It is my pleasure to announce my support for America's Mayor, Rudy Giuliani, a proven leader who is not afraid of what lies ahead and who will cast a hopeful vision for all Americans," Robertson said during a news conference with Giuliani in Washington.
The former New York mayor backs abortion rights and gay rights, positions that put him in conflict with conservative GOP orthodoxy, and has been trying to persuade evangelical conservatives like Robertson to overlook their differences on those issues.
Evangelicals have split in their support for the leading Republican candidates. Kansas Sen. Sam Brownback, a favorite of Christian conservatives who dropped out of the race last month, on Wednesday endorsed fellow Sen. John McCain of Arizona. Former Massachusetts Gov. Mitt Romney recently announced that Paul Weyrich and Bob Jones III were on board with his candidacy.
Asked about the Robertson endorsement, McCain, at a news conference with Brownback in Dubuque, Iowa, said: "Every once in a while, I'm left speechless. This is one of those times."
Giuliani is best known for leading New York in the aftermath of the Sept. 11 terrorist attacks. Shortly after 9/11, Robertson released a statement in which he said the attacks occurred because Americans had insulted God and lost the protection of heaven by allowing abortion and "rampant Internet pornography."
Robertson made no mention of his differences with Giuliani on social issues in Wednesday's statement.
"Rudy Giuliani took a city that was in decline and considered ungovernable and reduced its violent crime, revitalized its core, dramatically lowered its taxes, cut through a welter of bureaucratic regulations, and did so in the spirit of bipartisanship which is so urgently needed in Washington today," Robertson said.
Robertson, who unsuccessfully ran for president in 1988, founded the Christian Broadcasting Network, the Christian Coalition and Regent University in Virginia Beach, Va.
Giuliani said Wednesday he got to know Robertson well on a flight from Israel.
"I came away from it with a better understanding of Pat, what he's all about, what he's trying to accomplish," he said. "And I think he came away with a different impression of me, as well. We see the world, in many ways, the same way. Doesn't mean we agree on everything."
Also Wednesday, Giuliani said he asked two GOP friends in Congress, Reps. Peter King of New York and Rep. Pete Sessions of Texas, to introduce bills to keep states from giving driver's licenses or similar identification to illegal immigrants.
The Democratic front-runner, Hillary Rodham Clinton, was criticized after a televised debate last week when she hedged an answer on whether she supported New York Gov. Eliot Spitzer's effort to grant licenses to illegal immigrants. Her aides say she generally supports the idea in the absence of comprehensive immigration reform.
Associated Press Writer Amy Lorentzen in Dubuque, Iowa, contributed to this report.