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Senator John McCain announced his intention to run for president on the Late Show with David Letterman Wednesday night. ((CBS/Associated Press))

Republican Senator John McCain plans to make a formal announcement of his presidential candidacy next month after a trip to Iraq.

The Arizona senator discussed the early-April timing of the long-expected announcement with reporters Wednesday evening a few hours after taping an appearance on CBS' Late Show with David Letterman.

McCain told Letterman: "The last time we were on this program, I'm sure you remember everything very clearly that we say, but you asked me if I would come back on this show if I was going to announce.… I am announcing that I will be a candidate for president of the United States."

He told reporters later that he would visit Iraq before his formal announcement and that his campaign would be about "whether I have the vision, experience and knowledge to lead the nation."

Discussing the war with Letterman, McCain repeated his assertion that U.S. troops must remain in Iraq rather than withdrawing early even though the war has been mismanaged.

"Americans are very frustrated, and they have every right to be," McCain said. "We've wasted a lot of our most precious treasure, which is American lives."

In February, Democratic presidential candidate Barack Obama described the lives of troops in Iraq as having been "wasted" but then apologized a day later for making what he called "a slip of the tongue" that he said was not meant to diminish their sacrifice.

Expected to make candidacy official in spring

Former New York governor George Pataki introduced McCain to reporters before the Irish-American 10th Annual Awards reception Wednesday night. Asked about polls showing him trailing former New York mayor Rudy Giuliani, McCain said: "We keep doing the best we can. We're very happy with the way things are going."

There had been no doubt that McCain would eventually become a full-fledged White House candidate, and he had been expected to make his candidacy official in the spring.

The 2006 mid-term campaign had just ended when McCain took the first formal step toward a presidential run in November. He formed an exploratory committee and gave a speech casting himself as a "common-sense conservative" in the vein of Ronald Reagan who could lead the party back to dominance.

McCain faces strong challenges from Giuliani, who has widened his lead over McCain in popularity polls in recent weeks, and former Massachusettsgovernor Mitt Romney, who is little-known nationally but is drawing notice for his deft fundraising.

Go-to guy on Iraq

McCain, a former Navy pilot who was a prisoner of war in Vietnam, has emerged as the Senate's go-to guy on Iraq. He has become President George W. Bush's most outspoken supporter of sending 21,500 additional U.S. troops to Iraq.

A four-term senator, McCain unsuccessfully ran for president in 2000 against Bush and has been laying the groundwork for a second run for more than a year.

Should he win this nomination and then the presidency, McCain, 70, would be the oldest president ever sworn into office for a first term. Only Reagan, who was 73 at the start of his second term, was older.

Asked by Letterman if he would consider being a vice-presidential candidate, McCain repeated an answer he gave in 2004 when he was mentioned as a possible running mate: "You know, I spent all those years in a North Vietnamese prison camp, kept in the dark, fed scraps — why the heck would I want to do that all over again?"