Democratic presidential challenger Mr John Kerry has accused US President George W. Bush of using surrogates to "do his dirty work".
Mr Kerry said a group called the Swift Boat Veterans for Truth, which has attacked the Massachusetts senator's war record via television advertisements, was funded by hundreds of thousands of dollars from a Republican contributor in Texas, Mr Bush's home state.
"They're a front for the Bush campaign. And the fact that the president won't denounce what they're up to tells you everything you need to know: he wants them to do his dirty work," Mr Kerry told a convention of the International Association of Fire Fighters, which backs Mr Kerry.
Bush campaign spokesman Mr Steve Schmidt said the president did not support the Swift Boat ads, which accuse Kerry of lying about his Vietnam combat record.
"John Kerry knows his statements are false. John Kerry knows the president said his service in Vietnam was noble," said Mr Schmidt. He said Mr Bush wants an end to all political advertisements paid for by so-called soft money.
Mr Kerry has been put on the defensive by weeks of Republican criticism of his Vietnam War service as a decorated navy lieutenant. American military veterans usually vote in higher numbers than the general population, and Democrats fear the criticism may hurt his standing in key states in the November election.
In addition, the Iraq war and fear of terrorism have given foreign policy and national security greater importance than economic issues in this presidential election, for the first time since the Vietnam era, the Pew Research Center said in a poll released Wednesday.
"Of course, the president keeps telling people he would never question my service to our country. Instead, he watches as a Republican-funded attack group does just that," Mr Kerry said.
"Well, if he wants to have a debate about our service in Vietnam, here is my answer: 'Bring it on,'" the Massachusetts senator said, to cheers from the firefighters.