In a preview of their likely stances on foreign policy if they end up going head to head for the White House, Mr McCain painted a picture of Mr Obama as being timid, inexperienced and naive, while the Democrat portrayed the older man as bellicose and out of touch.

Mrs Clinton was not mentioned despite the fact that - like Mr Obama - she wants to pull US troops out of Iraq.

At 71, Mr McCain would be the oldest person elected to a first presidential term, while Mr Obama, 46, would be one of the youngest, and their war of words highlighted their difference in age and experience.

The first shots were fired by Mr McCain, who mocked Mr Obama for saying in Tuesday night's Democratic debate that after US troops were withdrawn, he would act 'if Al-Qaeda is forming a base in Iraq'.

The Republican told a crowd in Texas: 'I have some news. Al-Qaeda is in Iraq. It's called 'Al-Qaeda in Iraq'.'

Bloomberg rules out bid NEW York Mayor Michael Bloomberg has ruled out an independent bid for the White House, averting what could have become a major upset to a two-horse presidential race. 'I listened carefully to those who encouraged me to run, but I am not - and will not be - a candidate for president,' he wrote in a New York Times op-ed piece. ... more NEWSFLASH 'I have some news. Al-Qaeda is in Iraq. It's called 'Al-Qaeda in Iraq'.' MR McCAIN, mocking Mr Obama for saying that he would act if Al-Qaeda was forming a base in Iraq

In a toughly worded attack, he also accused Mr Obama of replacing his signature appeal for 'the audacity of hope' with 'the timidity of despair' when it comes to Iraq, and saying that a withdrawal would amount to surrendering to Al-Qaeda.

Mr Obama, who was campaigning in Ohio, quickly shot back: 'I have some news for John McCain - there was no such thing as Al-Qaeda in Iraq until George Bush and John McCain decided to invade.

'They took their eye off the people who were responsible for 9/11 and that would be Al-Qaeda in Afghanistan, that is stronger now than at any time since 2001.'

He said he intended to withdraw US forces from Iraq to concentrate on rooting out Al-Qaeda in Afghanistan and Pakistan.

And driving home his point, the Illinois senator dwelt on Mr McCain's stated determination to follow Al-Qaeda mastermind Osama bin Laden to the 'gates of hell' if need be, saying: 'So far, all he's done is follow George Bush into a misguided war in Iraq that has cost us thousands of lives and billions of dollars.'

Meanwhile, Mrs Clinton is counting on keeping her waning White House dreams alive by winning next Tuesday's Ohio and Texas primaries.

Left out of the clash over Iraq, she attempted to focus voters' minds on domestic matters during a swing through Ohio, saying: 'Obviously, the economy is the No. 1 issue in the country, and it's unbelievably important here in Ohio.'

And in a further setback for Mrs Clinton, Democratic congressman and civil rights leader John Lewis has dropped his support for her to become the latest superdelegate to throw his weight behind Mr Obama's campaign.

Mr Obama currently has 1,375 normal delegates to Mrs Clinton's 1,277, with 270 more at stake in next week's votes.

With 2,025 needed to secure the nomination, the superdelegates - senior party members with powerful votes - could decide the race.

ASSOCIATED PRESS, AGENCE FRANCE-PRESSE, LOS ANGELES TIMES, REUTERS