Republican presidential contender John McCain, a staunch backer of the Iraq war but critic of how President Bush has waged it, said U.S. lives had been "wasted" in the four-year-old conflict.

Democrats demand the Arizona senator apologize for the comment as Sen. Barack Obama did when the Democratic White House hopeful recently made the same observation.

"Americans are very frustrated, and they have every right to be," McCain said Wednesday on CBS' "Late Show With David Letterman." "We've wasted a lot of our most precious treasure, which is American lives."

Click here for FOXNews.com's You Decide 2008 center.

McCain, who repeated his assertion that U.S. troops must remain in Iraq rather than withdrawing early, made the "wasted" remark after confirming to Letterman what has been clear for at least a year or more — that he's in the running for the 2008 Republican nomination.

"I am announcing that I will be a candidate for president of the United States," he said — and added that he would officially enter the race by giving a formal announcement speech to that effect in April after a visit to Iraq.

Hours after the taped appearance aired, the Democratic National Committee called on McCain to take back the "wasted" lives remark.

"Senator McCain should apologize immediately for his callous comments," said Karen Finney, a DNC spokeswoman. "How is it that John McCain now believes American lives are being wasted, yet he so stubbornly supports the president's plan to escalate the war in Iraq and put more American lives in harms way?"

A message seeking comment was left with McCain's campaign.

In February, Obama described the lives of troops in Iraq as having been "wasted," but then later said he regretted the comment.

"We ended up launching a war that should have never been authorized, and should have never been waged, and on which we've now spent $400 billion, and have seen over 3,000 lives of the bravest young Americans wasted," Obama, D-Ill., said at a rally in Ames, Iowa.

Later, in an interview with a newspaper reporter, Obama said: "I was actually upset with myself when I said that, because I never use that term."

"Their sacrifices are never wasted," he said.

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. There had been little doubt that he would become a full-fledged White House candidate. He had been expected to make his candidacy official in the spring.

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

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

"We keep doing the best we can. We're very happy with the way things are going," McCain told reporters Wednesday in response to a question about him trailing Giuliani in polls.

Giuliani and Romney have spent the past two months mostly campaigning while McCain largely has been tied to Capitol Hill in his role as the highest-ranking Republican on the Senate Armed Services Committee, which is focused largely on the unpopular Iraq war.

A former Navy pilot who was a prisoner of war in Vietnam, he has emerged as the Senate's go-to guy on Iraq. In recent months, McCain has become Bush's most outspoken supporter of sending in another 21,500 troops to Iraq — even though he for years has leveled strong criticism about how Bush has handled the war.

Click here for FOXNews.com's You Decide 2008 center.