A lawyer for the Bush re-election team has resigned after admitting links to a veterans' group that accuses rival John Kerry of lying about his war record.
Benjamin Ginsberg said he did not want to distract attention
Benjamin Ginsberg advised the Swift Boat Veterans for Truth group, which ran TV advertisements questioning Mr Kerry's bravery in the Vietnam War.
The war records of President Bush and his Democrat opponent Mr Kerry have become a key issue in the campaign.
This week Mr Bush called for to an end to unofficial, negative advertising.
The president praised Senator Kerry's war record, saying he had served "admirably".
Mr Ginsberg told Mr Bush in a letter that he had decided to resign as his "legal representations" had become a distraction "from the critical issues at hand in this election".
He wrote to Mr Bush that he wanted to ensure "that the giving of legal advice to decorated military veterans, which was entirely within the boundaries of the law, doesn't distract from the real issues upon which you and the country should be focusing".
Senator Kerry's team has accused the self-proclaimed non-partisan Swift Boat Veterans for Truth of being a front for the Republican re-election campaign.
A spokesman for the Kerry camp, Chad Clanton, said Mr Ginsberg's announcement was "another piece of the mounting evidence of the ties between the Bush campaign and this group".
The Bush team has repeatedly denied links with the group.
"There has been no co-ordination at any time," Bush campaign spokesman Scott Stanzel said.
Correspondents say Senator Kerry has made his service in Vietnam - where he earned five medals - a cornerstone of an election campaign dominated by concerns over national security, traditionally seen as a Republican strength.
The Swift Boat Veterans for Truth advertisements alleged Mr Kerry exaggerated his valour in combat.
Mr Kerry's team has released a new set of ads in which the men who served under him on a naval vessel in Vietnam pay tribute to his bravery.