US Senator John McCain has said he will officially enter the race to be the Republican candidate for the presidency in 2008.
The Vietnam war veteran lost a previous bid for his party's nomination in 2000 to President George W Bush.
He has been a strong supporter of the Iraq war and was an early advocate of sending in more US troops.
Opinion polls suggest the former mayor of New York, Rudolph Giuliani, is the favourite candidate among Republicans.
Mr McCain, 70, made the announcement during a pre-recorded interview for CBS's Late Show with David Letterman, broadcast late on Wednesday.
"I am announcing that I will be a candidate for president of the United States," he said.
Mr McCain told David Letterman that his formal announcement would be made in April.
"You know, you drag this out as long as you can. You don't just have one rendition," he told Mr Letterman.
Pressed by Mr Letterman, the Arizona senator joked: "This is the announcement preceding the formal announcement!"
He formed an exploratory committee last November and since then has been making public appearances throughout the US.
A recent opinion poll shows that the once frontrunner is now trailing Mr Giuliani - who became known as "America's Mayor" for his handling of the 11 September 2001 attacks on New York - by two to one among Republicans.
Senator McCain has long been known as a straight-talking maverick, says the BBC's James Westhead in Washington.
Although a strong supporter of the war in Iraq, he has sometimes been critical of the Bush administration's handling of the war.
He has also been critical of former Defence Secretary Donald Rumsfeld, who, he said, would go down as one of the worst in history.
Mr McCain was an early supporter of sending more troops to Iraq - even in the aftermath of the November midterm Congressional elections when withdrawal was a more popular option.
He has challenged the Republican Party over corporate financing and been critical of the White House over torture and so-called renditions of terror suspects.
A Vietnam veteran, Mr McCain served five years as a prisoner of war, after being shot down as a Navy pilot in 1967.
Our correspondent says his age may be Mr McCain's biggest handicap. If he won the presidency, he would by then - at 72 - be the oldest man ever elected to the White House.