McCain, who lost his party's nomination in 2000 to now-President George W. Bush, made the announcement on an unusual venue: CBS television's "Late Show with David Letterman," a popular comedy show.

"I am announcing that I will be a candidate for president of the United States," McCain said on the show.

He later said that he will be making the formal announcement in April. "You drag this out as long as you can," he said, half-joking. "You don't just have one rendition."

The outspoken Arizona senator, 70, once a front-runner among Republican faithful, has seen his standing in the polls slip. A Wednesday survey placed him well behind his main rival, former New York mayor Rudolph Giuliani.

McCain has long supported the unpopular Iraq war, arguing that more soldiers were needed to flush out insurgents, crush militias and train Iraqi forces -- a position since taken up by the president.