The controversy over John F. Kerry's service in Vietnam and his days as an antiwar protester entered its third week yesterday with both Kerry and President Bush vulnerable to the political fallout from an episode that has unexpectedly come to dominate the coverage of the presidential campaign.

Privately, key Democratic strategists fear that attack ads against Kerry will undermine the Democratic presidential nominee's character and credibility, no matter whether the charges are accurate, because they dovetail with an argument Bush's campaign has tried to pound home in its advertising -- that Kerry is unreliable and untrustworthy. Many of the charges have been rebutted by veterans who served with Kerry and by military records.

But some Republican strategists see the potential for a backlash developing that will hurt the president. They say Bush must overcome doubts about his leadership on Iraq and the economy, and can ill afford voters concluding that he and his campaign orchestrated the attacks on Kerry by Swift Boat Veterans for Truth.

"This potentially undercuts the Kerry message by damaging the messenger," said one GOP strategist, who declined to be identified because of concerns about retribution from Bush's advisers. "But if the election is fundamentally about Bush, I don't see how his cause is advanced" by a debate about Vietnam.

Whether the argument over Kerry and Vietnam turns out to be an August diversion or a significant moment in one of the most fiercely fought campaigns in modern presidential history is not clear to either side. There is little reliable polling to suggest an answer -- what is available shows no significant change in the race -- but the Kerry campaign has responded, belatedly in the eyes of some Democratic allies, as if the charges have the potential to significantly damage the senator from Massachusetts.

As the Swift boat group launched a second television commercial yesterday, this one focused on Kerry's antiwar activities, the candidate's advisers expressed optimism that their efforts to fight back have helped them turn a corner. Calling the battle "a big fight we need to wage and win," adviser Tad Devine said: "When these issues and questions about his service are raised and answered with the truth, the people who raise them are hurt, and John Kerry is strengthened."

Bush advisers see Kerry's decision to fight back as a sign that the attacks were undermining his candidacy -- and a hindrance to his efforts to give voters a reason to vote Bush out of office. "Every day they can't make an argument about why not us and why him is not a good day for a challenger," one Bush strategist said.

Democratic strategists outside the Kerry campaign worry that the Swift boat ads undermine the foundation of the challenger's campaign -- his biography and particularly his service in Vietnam. These strategists say they believe that the attacks have hurt Kerry and that, like charges in 2000 that Vice President Al Gore was prone to exaggeration, they could become part of a continuing story line surrounding his candidacy -- reinforced by Bush's advertising for the past five months.

Furious at what they call unfair attacks, these Democratic strategists see them as effective because of potential questions about Kerry's credibility. Kerry's campaign has been forced to backtrack on his statements that, while in Vietnam, he engaged in combat in Cambodia on Christmas 1968, suggesting he was part of an illegal war there authorized by President Richard M. Nixon, who had not been inaugurated.

Other Democrats say that in the end, the fact that Kerry served voluntarily in Vietnam will be enough to satisfy swing voters, who are more worried about Iraq and the economy than about what happened 35 years ago.

But even some Democrats who agree that Kerry's biography is the key to his hopes of winning saw the Democratic National Convention in Boston as going too far in emphasizing Vietnam.

Republicans say the convention's focus invited even greater scrutiny of Kerry's record and what he has had to say about it. "Kerry brought this upon himself by making the convention all about his military service," said Sen. Rick Santorum (R-Pa.). "He stuck his jaw out on this one and now has to deal with the consequences."

Still, Republicans say Kerry's protests after his Vietnam service could hurt him the most in the election. When Kerry returned from Vietnam, he burst onto the national stage as a leader of the antiwar movement, a well-spoken critic of U.S. policies in Vietnam. Most famously, he testified in 1971 before the Senate Foreign Relations Committee, where he enumerated war crimes and atrocities that he said were committed by U.S. troops in Vietnam.

In an interview yesterday, former senator Robert J. Dole (R-Kan.) said he recently went back and reread that testimony and viewed the television debates aired that year on "The Dick Cavett Show" between Kerry and John E. O'Neill, author of a new book condemning the Democratic nominee's war service. "It troubled me then, and it troubles me now," he said.

Dole said Kerry's "grandstanding" just didn't "smell right." Swift Boat Veterans for Truth's new ad criticizes Kerry's 1971 testimony, including his comments that soldiers reported how Americans raped and beheaded the enemy.

Rep. Mike Rogers (R-Mich.), a veteran, said this line of attack is fair game. "What does offend me, and what I think is unconscionable, was when we had soldiers in the field and people who were prisoners of war, he labeled them all . . . as war criminals," he said. "I still get mad about that some 30 years later."

Scott Reed, who managed Dole's 1996 presidential campaign, said the controversy has derailed progress Kerry was making on becoming an acceptable alternative to Bush as commander in chief. Although Kerry advisers say the damage is slight, they acknowledge that the Democrat's post-convention gains on the question of who is better able to serve as commander in chief have been eroded, whether by natural deflation or the Swift boat veterans' attacks.

Given the potential problems, and spurred by Kerry and his Vietnam crewmates, who argued it was time to respond, the campaign has moved aggressively on several fronts. It has assembled evidence and eyewitnesses to rebut the attacks and, just as significantly, has tried to pin the blame on Bush with a new TV ad.

Kerry advisers hope to put Bush on the defensive by suggesting a pattern to his campaigns; they argue that Bush allies sought to sully the military record of Sen. John McCain (Ariz.) during the 2000 GOP primary in South Carolina and that the GOP did the same to then-Sen. Max Cleland (D-Ga.) two years ago. The Kerry camp also has called on Bush to condemn the Swift boat ad, noting that Kerry denounced a scurrilous ad run by a Democratic group.

"Making the connection to the president is very powerful to voters because it opens a whole line of scrutiny around the president and how he has dealt with this issue before," Devine said.

Bush advisers deny a link between the campaign and Swift Boat Veterans for Truth, and continued to maintain that stance last night after a report that the campaign's chief outside counsel, Benjamin L. Ginsberg, had provided legal advice to the veterans. "At no point was there ever any coordination between our campaign and any of Ben's clients," said campaign communications director Nicolle Devenish.

Bush advisers see hypocrisy at work, contending that Kerry has called on Bush to denounce the Swift boat group for spending a few hundred thousand dollars on two negative ads while Kerry has stood by as Democratic groups have spent tens of millions on negative ads aimed at the president, condemning just one. "It's too bad that 46 ads later Kerry called for one to be taken down," said Matthew Dowd, chief strategist for the Bush-Cheney campaign.

With the Republican National Convention next week, it is possible that the Vietnam controversy will quickly fade, and some strategists say swing or undecided voters will be happy to see it go. "This is a debate designed for partisans, or at least is satisfying to the partisans on one side and enraging the partisans on the other," said Geoffrey Garin, who is polling for the Democratic National Committee. "But it is close to irrelevant to the swing voters in the middle."

Even if that is the case, Kerry's campaign has decided it cannot take the chance of not fighting back.