Vietnam veterans opposing John Kerry (search) have scored a hit with a tough TV ad that claims he lied about his war record — it makes swing voters think twice about backing Kerry, an independent study has found.

The ad planted doubts in the minds of 27 percent of independent voters who planned to vote for Kerry or leaned pro-Kerry. After seeing it, they were no longer sure they'd back him, the study found.

At issue is the ad run by an independently funded group known as Swift Veterans for Truth, which features 14 veterans who served in Vietnam along with Kerry as they accuse him of lying about his war record.

"The whole goal of a negative ad is to plant a seed of doubt — and it did," said Professor Chris Borrick of Muhlenberg College, who did the ad study with the firm HCD Research.

"Obviously, the Swift Boat Veterans' intent was to publicly question Kerry's war record, and it looks like they succeeded," said HCD chief Glenn Kessler, who added that viewers found the 60-second ad more believable as it went on.

Kessler, who wasn't paid by anyone for the analysis, said he leans Democratic.

The Swift Vets study used 1,275 participants, including 371 independents, who watched ads and registered their reaction at every second using technology normally used to rate product ads. Half viewed the Swift Vets ad and the other half saw a pro-Kerry ad based on his convention speech, which was rated less persuasive.

HCD is also completing a study of a counter-ad by the liberal group MoveOn.org that questions President Bush's military record — it appears far less effective in raising doubts among pro-Bush swing voters.

Kerry yesterday issued a statement condemning MoveOn's anti-Bush ad as "inappropriate" after Sen. John McCain blasted it — but a key Kerry ally, retired Gen. Wesley Clark, echoed the MoveOn charges that Bush ducked combat duty.

One of the anti-Kerry vets, John O'Neill, wrote the best seller "Unfit for Command," which lambastes Kerry as a cowardly liar and says his claim to have spent Christmas 1968 on a covert mission to Cambodia was a lie.

The Kerry camp is now hedging on that claim, which Kerry once told the Senate was "seared" into his memory ever since President Richard Nixon falsely claimed there were no U.S. troops in Cambodia. In fact, Nixon wasn't yet president.

The Democrat's team yesterday held a press conference of pro-Kerry vets, but none of his crewmates was able to validate his Christmas-in-Cambodia claim.