McCain, who lost his party's nomination in 2000 to George Bush, is a former Vietnam prisoner of war who stresses his experience in foreign policy and military affairs.

After his appearance on the programme, the Arizona senator told reporters he would officially enter the presidential race with an announcement after a visit to Iraq in April.
 
More troops

McCain advocated sending more troops to Iraq even before Bush's unpopular announcement of a "surge" of 21,500 US soldiers to improve security in Baghdad.
 
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"Without additional forces, we cannot win this war," he said in the wake of the Republicans' loss in November congressional elections.

But, despite his support for the conflict, he has repeatedly criticised the way it has been conducted.

In November, McCain used a congressional hearing to dress down General John Abizaid, the US commander in the Middle East.
  
"I regret deeply that you seem to think the status quo and the rate of progress we're making is acceptable. I think most Americans do not," he said.

And during a speech in February, McCain, the highest ranking Republican on the senate armed services committee, attacked Donald Rumsfeld as "one of the worst defence secretaries in history".

"We are paying a very heavy price for the mismanagement - that's the kindest word I can give you - of Donald Rumsfeld of this war," he said. "The price is very, very heavy and I regret it enormously."

Republican field

McCain joins Rudolph Giuliani, the former New York mayor, and Mitt Romney, the former Massachusetts governor, who are leading a large field of Republicans eyeing a 2008 White House run.

McCain said on Wednesday that his campaign would be about "whether I have the vision, experience and knowledge to lead the nation".
 
A new ABC News/Washington Post poll shows Giuliani holding a two-to-one advantage over McCain among Republicans nationally.

Asked about the polls, he said: "We keep doing the best we can. We're very happy with the way things are going."