BOSTON (Reuters) - John Kerry accused President Bush of using surrogates to "do his dirty work," as the Democratic presidential challenger hit back on Thursday at a Republican assault on his Vietnam War record.

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, 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," Kerry told a convention of the International Association of Fire Fighters, a politically active union which backs Kerry.

Bush campaign spokesman 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 Schmidt. He said Bush wants an end to all political advertisements paid for by so-called soft money.

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," 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.

MILITARY STRENGTH

Kerry has made frequent references to his Vietnam War experience during his campaign to make the case for his strength as a military leader. His supporters have contrasted his record with that of Bush, who spent the war in the United States serving in the Texas Air National Guard.

Iowa Sen. Tom Harkin, a Democrat, has also called Vice President Dick Cheney a "coward" for attacking Kerry's toughness despite his own lack of military service during the Vietnam War.

A report on Thursday in the Washington Post said military records contradicted a claim that Kerry did not come under fire during the battle that resulted in military honors for the Democratic senator.

Larry Thurlow, who commanded a Navy boat alongside Kerry in Vietnam, has disputed that Kerry was shot at during the March 1969 raid that resulted in Bronze Stars being awarded to both men. Thurlow is a member of Swift Boat Veterans for Truth.

Thurlow's military records, partially obtained under the Freedom of Information Act by the Washington Post, contain references to "enemy small arms and automatic fire" directed at "all units" of the five-boat flotilla, the newspaper says.

Kerry noted that 30 years ago, official Navy reports documented his service in Vietnam and awarded him the Silver Star, the Bronze Star and three Purple Hearts.