Washington, D.C. ? Sen. John McCain of Arizona declared Wednesday night that he will join the 2008 race for the White House and will formally announce his candidacy in April.

McCain used an appearance on CBS’s “Late Show With David Letterman” to say what has been clear for many months, erasing whatever doubts may have existed that he intends to battle for the Republican nomination, which eluded him in 2000.

“I am announcing that I will be a candidate for president of the United States,” the former Navy pilot and Vietnam War prisoner told Letterman.

McCain’s decision to use the program to declare his intentions followed a pattern increasingly common in this presidential contest, as candidates have used multistep announcement schedules to garner maximum attention for their bids.

In this case, however, McCain, 70, may have additional motives for using the late-night comedian’s show, as he tries to rekindle some of the spontaneity and unpredictability from his first campaign. He cast himself as an insurgent politician in 2000, but this time, weighed down by a supportive position on the Iraq war that is out of step with the public even as he methodically woos the GOP establishment, he has struggled to project the buoyant personality of his first effort.

McCain lost a bitter contest for the Republican nomination to George W. Bush in 2000. He emerged as the early leader in the race for the 2008 GOP nod, in part because of his support for the president’s leadership on the Iraq war but also because he has spent months courting Bush loyalists and the Republican establishment that had spurned him.

In recent months, however, his star has been eclipsed somewhat by that of former New York mayor Rudolph Giuliani, who now leads McCain in many national and some state polls testing the Republican field. A Washington Post-ABC News poll released Tuesday showed Giuliani leading McCain 44 percent to 21 percent. A month ago, Giuliani’s advantage was much narrower, at 34 percent to 27 percent for McCain.

McCain advisers said the decision to declare that he will join the Republican race was not a direct result of concern that Giuliani has gained ground in the past two months but rather part of a long-planned strategy to make his intentions known around this time.