After nearly five months in which President Bush's ads have portrayed their man as a flip-flopping, tax-raising, soft-on-defense lawmaker who would fail to protect American troops and pregnant women, John F. Kerry's strategists have formulated a response.

It is embodied in an ad featuring running mate John Edwards, who says that anyone who doubts Kerry is a leader should "just spend three minutes with the men who served with him 30 years ago."

"That spot is intended to inoculate against the repeated attacks on Kerry's character," said Tad Devine, a senior strategist for the Massachusetts senator's presidential campaign. "The calculation on our part was that the best single answer was not a specific rebuttal but to provide a broad context on who Kerry is. We don't have to join their game."

Ever since another Massachusetts presidential candidate, Michael S. Dukakis, passively allowed Bush's father to negatively define him in 1988, the Democratic mantra has been that failure to respond to Republican attacks is a recipe for disaster. But while Kerry aides have challenged the accuracy of Bush's charges, the senator's ads in recent months have ignored those charges and delivered a largely positive message.

Kathleen Hall Jamieson, dean of the Annenberg Public Policy Center at the University of Pennsylvania and co-author of "The 2000 Presidential Election and the Foundation of Party Politics," said that what might be called Kerry's Vietnam War defense is adequate. "You don't have to engage that at all," she said of the Bush attacks. "All you have to say is, look at this moment in his biography where the guy is shot and Kerry turns the boat around."

Mark McKinnon, Bush's media adviser, said his team's television spots have had "as much impact as possible in a presidential campaign." He said polls show that substantial numbers of voters now believe the "two primary themes we've advertised on . . . that Kerry is not a decisive leader who is political and lacks core convictions, and he has a history of raising taxes significantly and is likely to do so in the future."

McKinnon disputes the notion that his side's $85 million ad effort has been unusually negative. "Over the course of the campaign, our advertising will not vary from any incumbent presidential campaign," he said.

In 1984, McKinnon said, President Ronald Reagan aired 18 of 50 of his ads against his opponent, Walter F. Mondale. In 1996, McKinnon said, seven in 10 ads by President Bill Clinton and the Democratic National Committee contained negative information about GOP nominee Robert J. Dole. By McKinnon's count, 10 of the Bush campaign's 18 spots have attacked Kerry. But as much as 75 percent of Bush's ad traffic has been on the negative side, according to one industry analysis.

Devine said his campaign's focus groups found many voters concluding from the Bush television attacks that the president "had no plan on the issues important to their lives. There was a collective rolling of the eyes."

While the Bush camp's early flip-flop ads made a mark, Devine said, it has faded. "They thought this time period would be like Clinton in '96, the incumbent president defining and destroying his opponent," he said. But the Bush blitz "has not succeeded in poisoning the well."

That is in part because Kerry has raised more than $100 million since effectively clinching the nomination in March, allowing him to keep pace with the president. In 1996, the Dole campaign was broke for months while Clinton and the DNC hammered him on the air.

The Bush ads have, among other things, blamed Kerry for voting "against the Laci Peterson law -- the law protecting pregnant women from violence." Other spots accused Kerry of having "repeatedly opposed weapons vital to winning the war on terror," and voting "for the Patriot Act, but pressured by fellow liberals, he's changed his position."

Kerry's ads, by contrast, have pushed proposals on the economy and health care. A new spot Tuesday urged "an end to tax incentives that are encouraging American companies to ship jobs overseas" and an investment in "alternative fuel sources." The most negative statement in a recent Kerry ad was that "John Kerry will fight to bring back the 1.8 million jobs that have been lost under George W. Bush."

Matthew Dowd, the president's chief campaign strategist, said Kerry aides "can act self-righteous" about positive ads "because they have his little buddy allies carrying the weight of negative attacks on the president." He was referring to independent liberal groups, such as MoveOn.org and the Media Fund, which have been financing a wave of anti-Bush ads. If these outside organizations are included, the Bush campaign has been outspent on the air.

McKinnon said that until Kerry campaign manager Mary Beth Cahill "stands up and admonishes her buddies, it is disingenuous and hypocritical for them to try and score points for running a positive campaign." Devine said the Kerry camp has no control over these outside groups, which by law are not allowed to coordinate their efforts with the senator's operation.

Jim Jordan, the former Kerry campaign manager who runs the Media Fund, said: "It was our calculation that we could be most effective by attacking Bush policies and the GOP agenda broadly. That's what seemed to move the needle the most."

Kerry advisers may have another tactical reason for staying positive during a period when the administration has been battered by headlines about Iraq and the Sept. 11 commission. "They don't have to attack the credibility of George Bush," Jamieson said. "The news has effectively done that already." Bush's "best bet is making sure Kerry never gets over the threshold required to be president."