Maverick Republican senator will give a boost to Kerry's campaign

Just possibly, John Kerry's most potent card in his quest for the White House is not the support of his fellow Democrats, nor his party's seething rage at President Bush, but his close friendship with a Republican, his Senate colleague John McCain of Arizona.

Just possibly, John Kerry's most potent card in his quest for the White House is not the support of his fellow Democrats, nor his party's seething rage at President Bush, but his close friendship with a Republican, his Senate colleague John McCain of Arizona.

Mr McCain, 66, is not just the underdog challenger who gave Mr Bush a serious fright in the 2000 primaries. He is one of the most popular - some would say the most popular - politicians in the country, a maverick with a powerful appeal to independent voters.

Right now, he is also an all-important defender of Mr Kerry's national security record, which is under savage assault by the Bush-Cheney re-election machine.

Mr McCain and Mr Kerry are decorated Vietnam war heroes. Their friendship dates from 1991, when they worked together on a Senate committee looking into the fate of American soldiers missing in action in South-east Asia. Later, they pushed hard for the normalisation of relations between the US and Vietnam, which was finally achieved under President Bill Clinton.

Last week, the personal bond between the two men proved priceless for Mr Kerry, as Bush-Cheney attack advertisements zeroed in on the Massachusetts senator's alleged propensity to "flip-flop" on key defence issues, thus making him unfit to be commander-in-chief.

Not so, according to Mr McCain. His old friend had put together a "strong record" on national security during his 20 years in the Senate, Mr McCain said on 18 March. Republican strategists watched in frustration as he partly neutralised a blistering attack on Mr Kerry, which had been delivered just 24 hours earlier by Dick Cheney, the vice-president.

Political professionals on both sides of the fence agree that with Mr McCain in Mr Kerry's corner, the future Democratic nominee is less at risk from Mr Bush's greatest single asset in his re-election bid: his handling of the "war on terror". Republicans have just one consolation: it could have been worse. A week before springing to Mr Kerry's defence on national security, Mr McCain fleetingly left the door ajar to the even more tantalising possibility that he might run for vice-president on the Democratic ticket.

A few hours later, McCain aides slammed the door shut. But the notion, however short-lived, had a beguiling logic. After their fierce 2000 primary battle, there is no love lost between the President and Mr McCain.

Since Mr Bush took office, Mr McCain opposed the more extravagant of Mr Bush's tax cuts and sponsored a campaign finance reform bill which had been long resisted by the White House. When Mr Bush relented, he pointedly snubbed his former rival by not inviting him to the formal signing ceremony.

Those close to Mr McCain insist that his support for Mr Kerry is not a case of payback against the White House, but a sign of how he places loyalty to friends above loyalty to his party. For Republicans, the spectacle is awkward. A senior, very popular senator is breaking ranks with the President of his own party on a crucial election issue.

In an increasingly polarised climate on Capitol Hill, friendships across the aisle are increasingly rare.