This early stage of the presidential race has come down to arm-wrestling over the rights to the word "strength."

President Bush's greatest political asset has been his reputation as a tenacious, tough-minded executive who fights unflinchingly for his goals. In a Pew Research Center poll released last week, half the respondents said that Bush has the best claim to the term "strong leader"; only three out of 10 said it better fits Democratic challenger John Kerry.

That Bush advantage has survived two months of chaotic fighting, rising casualties and prison abuse scandals in Iraq and mixed messages on the economy, where job numbers are improving but gas prices are soaring, inflation is stirring and interest rates are on the rise.

The adverse developments have affected Bush's ratings on job approval and on broader measures of the national mood, dropping him to levels that historically have meant an incumbent president is highly vulnerable to defeat.

But so far these developments have not damaged Bush's reputation as a strong leader.

To cement the advantage, the Bush campaign has spent record sums on a negative advertising assault on Kerry, designed principally to convince people who know little about the Massachusetts senator that he is an unreliable steward of the nation's safety and well-being. These ads depict Kerry as a political chameleon who shifts positions for expediency and flip-flops on issue after issue.

The ads have had some effect. In Andrew Kohut's Pew poll, 42 percent of those interviewed said the phrase "changes his mind too much" applies to Kerry. Bush was seen in mirror-image terms; two-thirds of the voters said he might be considered "stubborn."

In an interview the same day the Pew poll was released, Tad Devine, one of Kerry's media advisers, said, "We recognize that Bush had an opportunity to demonstrate strength after 9/11" and "we know that quality is important to voters."

As a result, Devine said, the effort by the Kerry campaign to show that its candidate also has steel in his backbone is "the heart of the message" in the two biographical ads now running in 19 battleground states. Thanks to a $25 million buy -- an unprecedented sum for a non-incumbent candidate at this early point in the race -- voters in those states are being told over and over that Kerry is no wimp or waffler.

Both ads hark back to his readiness to volunteer for Navy combat duty in Vietnam and to risk his own life to save his crew members. One ad has Kerry pledging to "build a stronger America." In the other, a narrator says the candidate has demonstrated "a lifetime of service and strength."

The courage that Kerry displayed in Vietnam cannot be challenged, but the swerves in his political career and policy positions are not figments of the Bush campaign's imagination. As Howard Dean pointed out during the Democratic primaries, Kerry voted against the 1991 Persian Gulf War but later applauded the ouster of Iraqi forces from Kuwait. And Kerry voted for the second showdown with Saddam Hussein but quickly became a harsh critic of Bush's policy in Iraq.

There are many such examples among the hundreds of votes Kerry has cast in his long Senate career.

Senators are not executives, free to set their own course. They have to respond to others' initiatives, and it often leaves a muddled trail. But there is a larger problem that makes Kerry's "strongman" ads a political gamble. It is rare in campaign history for a challenger to gain by claiming a quality that voters identify with the incumbent.

In 1992 Bill Clinton did not try to convince voters that he was the first George Bush's match in foreign affairs; instead he shifted the debate to the economy and health care. In 2000 this George Bush did not claim to be smarter than Al Gore; he was content to show himself more likable and more comfortable in his own skin.

Devine and other Kerry strategists say they don't have to beat Bush in the "strength index." They would be satisfied just to neutralize the president's current advantage and then move on to exploit his weaknesses in vital policy areas such as Iraq and the economy.

But by accepting that "strength" is the password to those more promising policy debates, the Kerry campaign in effect has agreed to fight on Bush's turf. They have the money now to make the ad war a much more even contest than it was in March and April.

Whether they have chosen the right battleground is another question.