Attacking the heart of Mr Kerry’s election campaign — his decorated heroism while a Swift boat commander in Vietnam — the group accuses the Massachusetts senator of lying about the incidents that led to him being awarded the first of his three Purple Hearts and his Bronze Star.

The allegations made by Swift Boat Veterans for Truth, who are airing the advertisements in three swing states, have been fiercely denied by both the Kerry campaign and the men who served under his command.

John McCain, the Republican senator and former Vietnam prisoner of war, also defended Mr Kerry.

Referring to an advertising campaign which cast doubt on his Vietnam service during his bitter primary battle against President Bush in 2000, Mr McCain said: “It was the same kind of deal that was pulled on me. The ad is dishonest and dishonourable.”

The anti-McCain advertisements were made by the same firm as the anti-Kerry one.

A spokesman for Mr Kerry also pointed out that $100,000 of the $158,000 raised by the group came from one source: Bob Perry, a longtime contributor to Republican campaigns.

The veterans’ group, which says it has 250 members, has disliked Mr Kerry for more than 30 years since his decision to protest against the Vietnam War after returning home.

The most controversial claim in the advertisement is that Mr Kerry lied about an incident on March 13, 1969, that won him a Bronze Star. Mr Kerry and men with him that day say he risked his life to pull a special forces officer, Jim Rassman, into the boat while under enemy fire.

Van O’Dell, a former naval gunner’s mate, says: “John Kerry lied to get his Bronze Star. I know — I was there.” But while saying there was no enemy gunfire at the time, Mr O’Dell concedes that he was on a different boat near by.

Mr Rassman, a former Republican who is now a major face of the Kerry campaign, called Mr O’Dell’s account “pure fabrication”. The Kerry campaign also convened a conference call in which two of his crewmates defended his service.

Lewis Letson, another member of the anti-Kerry group, has repeated accusations he made this spring that Mr Kerry did not deserve his first Purple Heart, a medal awarded for injuries suffered in combat.

“I know John Kerry is lying about his first Purple Heart because I treated him for that injury,” Mr Letson says on the advertisement, being aired in parts of Ohio, West Virginia and Wisconsin.

Mr Letson said it was a minor flesh wound caused by a ricochet off friendly fire. As on previous occasions, the Kerry campaign said another doctor signed Mr Kerry’s treatment record.

The White House disassociated President Bush from the advertisement. Steve Schmidt, a Bush campaign spokesman, said: “The Bush-Cheney campaign has never and will never question John Kerry’s service during Vietnam.”

But the Vietnam group’s attack highlighted the extent to which a myriad political organisations not officially affiliated with either campaign, or legally allowed to be, are doing much of their dirty work. Since Mr Kerry accepted his party’s nomination last week, he is limited to spending $75 million in public funds before election day, which gives him a month on Mr Bush, who does not accept the Republican nomination until September 2.

But a spate of commercials on Mr Kerry’s behalf have been broadcast this week, paid for by independent groups intent on Mr Bush’s defeat. To the voter’s eye, the advertisements look remarkably like Mr Kerry’s own campaign, in style and content. In fact they are made by the Democrat National Committee, which is running a $6.5 million advertising campaign in 20 battleground states; the Media Fund, running a $2.5 million campaign in five swing states; and the New Democratic Network, spending $500,000 on Spanish-language commercials in 11 cities.

The groups are legally barred from even consulting with the Kerry campaign. But it does not take a genius, after a Democrat convention dominated by Mr Kerry’s war record, to divine what Mr Kerry might like to say. Matthew Dowd, Mr Bush’s chief campaign strategist, has long said that they would be “outspent by Kerry and outside groups”.

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