Max Cleland also called on Bush to publicly denounce the group known as Swift Boat Veterans for Truth and said the president should also condemn them.
"These scurrilous attacks on John Kerry's credibility ... his courage, his valour, are false, and George Bush is behind it," said the former senator from Georgia on Wednesday.
The wheel-chair bound veteran - who lost three limbs in Vietnam - called on the president "to specifically condemn the recent attack ads and accompanying campaign which dishonour Senator John Kerry's combat record in the Vietnam War.
"These false charges represent the worst kind of politics," the letter said. "As veterans of the armed services, we ask that you recognise this blatant attempt at character assassination, and publicly condemn it."
Bush has described Kerry's Vietnam service as admirable but has declined to specifically condemn the ads by the Swift Boat veterans.
His campaign has denied any connection with the group behind the attack ads.
But a top lawyer for Bush's re-election campaign resigned on
Wednesday after disclosing he had provided legal advice to a group that produces the ads.
Benjamin Ginsberg is the second person working for the
Bush campaign to be linked to the group, called Swift Boat
Veterans for Truth.
Kerry campaign spokesman Chad Clanton said the disclosure
provided "another piece of the mounting evidence of the ties
between the Bush campaign and this group."