At first the Kerry campaign dismissed them as cranks. But with their slickly made ad and frequent appearances on cable TV and talk radio, charging that Kerry had lied to win his medals in Vietnam, the Swift Boat Veterans for Truth began making inroads. According to a poll taken by the University of Pennsylvania's Annenberg Center, more than half the people surveyed had seen or heard about the ad, and about half of independent voters found the ad to be believable. Kerry's campaign fired up his own veterans' machine to try to stop any slippage--especially in key swing states like Ohio, where there are 1.1 million vets.

No wonder Kerry has lashed out, accusing President Bush of using a "front group" to do his "dirty work." According to campaign aides, Kerry himself chose the words to denounce Bush. Kerry became particularly outraged, aides say, when he read that Bob Perry, a friend of Bush senior adviser Karl Rove, was doubling his $100,000 contribution to the Swift Boat vets to buy another ad. The Bush campaign denied any involvement with the Swift Boat vets, and Bush ignored Kerry's demand that he disavow their scurrilous attacks.

Obscured by all the political maneuvering is the truth of what really happened 35 years ago. An examination of one key incident--Kerry's rescue of a comrade--tends to support Kerry's version of events, though questions remain. Official military records certainly back Kerry, but medal citations are known for making combat seem more glorious than it usually is. On the other hand, the Swift Boat veterans are obviously bitter at Kerry for testifying before Congress in 1971 about alleged war crimes, and their own credibility has proved to be questionable.

As sailors who weren't on Kerry's boat tell the story of what happened on March 13, 1969, Kerry did nothing very heroic. That day Kerry was leading five boats back from a mission up the Bay Hap River. Encountering a fishing net across the canal, the boats split. Kerry and one other boat went through a gap near the right bank, and the other three boats headed through an opening on the left. Suddenly, the lead boat on the left, some 25 yards away from Kerry's boat, hit a mine and stopped dead in the water. All the American boats opened up with machine gun fire, hosing down the shoreline. The firing stopped after about 40 seconds, according to the Swift Boat vets, when they realized that no one was shooting back. Kerry, they say, had roared ahead and then turned back to pick up a soldier who had fallen off his boat. When Kerry pulled the man out of the river, they claimed, no one was shooting.

Del Sandusky, the man steering Kerry's boat, told a different version to NEWSWEEK. He says his boat was jarred by an explosion, probably from a rocket, knocking the soldier, Jim Rassmann, off the boat. Kerry was thrown against the bulkhead, injuring his arm. Sandusky says he could see muzzle flashes from the jungle and bullets skimming across the water. Sandusky says he can't remember if anyone was still shooting when Kerry pulled Rassmann from the river, but in any case, the boat was banged up and taking on water. An official report made available to NEWSWEEK shows windows blown out and the engine and steering damaged; it's unclear from the report when the damage happened.

The fight over Kerry's war record may broaden into a larger battle over groups, like the Swift Boat vets, who buy ads to influence the election. So-called 527s, tax-exempt political organizations, are free to spend as much as they want to advocate their views, as long as they are truly independent of the candidates and their campaigns. The Kerry forces have filed a complaint with the Federal Election Commission, charging the Bush campaign with illegal "coordination" with the Swift Boat vets. The Bush camp immediately whacked back, claiming that the Democrats covertly run a far larger overlapping network of 527s. In the long run, scrutiny of 527s could pose more problems for the Democrats. Organizations like MoveOn, which last week was airing an ad mocking Bush's spotty attendance record in the National Guard, have spent tens of millions of dollars to savage Bush. Trying to figure out whether any of these attacks are, in fact, coordinated by the campaigns may be as difficult as sorting out what really happened on the waterways of the Mekong Delta in the Vietnam War.