John Kerry has made his military service in Vietnam central to his campaign for president, even crisply saluting fellow Democrats at their recent party convention and reporting "ready for duty".
The Democrats hope Mr Kerry's military record will help blunt the usual advantage that the Republican Party enjoys on issues of national security, especially in this election in which the war on terror and war in Iraq has elevated the issue over domestic concerns.
John Kerry has made much of his Vietnam service
But a group with ties to Republican Party supporters in Texas is running ads calling into question Mr Kerry's heroism and criticising his anti-war activities after he came back to the US.
It has put Mr Kerry on the defensive and calls into question a central pillar of his campaign.
The Kerry campaign has filed a complaint with the Federal Election Commission saying the ad violated the law with "inaccurate ads that are illegally co-ordinated with the Bush-Cheney presidential campaign".
FactCheck.org was set up by the Annenberg Public Policy Centre of the University of Pennsylvania "to reduce the level of deception and confusion in US politics" by monitoring the accuracy of campaign ads.
Brooks Jackson, director of the project, said the group had done an exhaustive analysis of the ad and supporting materials and found there was no way to resolve the conflicting accounts surrounding Mr Kerry's military service.
There are discrepancies in both the pro- and anti-Kerry stories and in the Navy records, he says, and attributes this confusion to the fog of war.
The group airing the ad, Swift Boat Veterans for Truth, did receive $100,000 of its initial funding from Houston builder Bob Perry who has given millions to the Republican Party and its candidates, including President Bush and Majority Leader Tom DeLay.
But Mr Jackson said: "I think that Kerry goes too far when he calls Swift Boat Veterans for Truth a Bush front group."
But whether the ads are true or not may not matter.
In ads, "something can be untrue but effective, and something can be true but ineffective", said Zachary White, a political communications professor at the University of San Francisco.
"Voters don't always arrive at a decision on clear cut rational standards," he said, adding that advertisers know that voters or consumers often make decisions based on "gut instinct".
But the ads have put Mr Kerry in a difficult position, he added.
If he responds, some might see it as legitimising the criticisms. "But if something is told and retold in the form of an ad, it takes on kernels of truth," said Mr White.
"They have taken Kerry's strength and turned it into a possible weakness," he added.
And Mr Kerry has had to redirect resources to counter the attack that might have been used for promoting his economic plan or explaining his position on Iraq.
Bruce Newman, who advised the Clinton campaign ahead of the 1996 election, said he was surprised it had taken John Kerry so long to respond to the attack.
"These accusations are beginning to stick," he said.
He said he believed the issue would not go away soon and would play prominently in the first debate between Mr Kerry and Mr Bush in the autumn.
"This is going to be a dirty campaign," he said.
The Bush campaign will probably not try to exploit the issue directly, Mr Newman said, but will most likely look for discrepancies in John Kerry's public statements about his Vietnam-era service to use in ads that reinforce his image as indecisive and a "flip-flopper".
And the ads move the debate away from Mr Kerry's strengths on the economy and other domestic issues back to the war on terror to where President Bush is strong, he said.