For much of the past three years, Democrats typically have preferred to change the subject -- to the economy, to education, to virtually anything -- than try to battle President Bush over his handling of terrorism. Consistently, polls have shown a majority of the public believes that the president has been attentive and steadfast on this issue.

This history makes John F. Kerry's strategy a striking departure, as the Democratic presidential nominee has begun a full-throated campaign not to change the subject, but to win the argument.

Rather than questioning the timing of the administration's latest warnings about a possible al Qaeda attack, Kerry accepted the warnings as credible but said they highlight the weakness of Bush's record.

A variety of polls and public opinion analysts make it clear that the Democrat faces an uphill climb with this argument. But some strategists in both parties said that Kerry is hardly waging a war of choice: The latest warnings underscored how much the balance of the presidential campaign will be shaped by outside events and hovering public anxiety over a potential terrorist attack. Kerry, they said, must prove himself credible on an issue that could shadow the next three months -- even if he might prefer the race be dominated by jobs or some other blue-chip Democratic issue.

Kerry is not the only one tethered to events. Bush's numbers on terrorism have shown fluidity. The latest Washington Post-ABC News poll showed him leading 48 percent to 45 percent on whom voters say they "trust to do a better job handling terrorism."

A week earlier, a Post-ABC poll showed that gap had been 18 percentage points, with Bush leading by 55 percent to 37 percent. These data suggest that Kerry succeeded in one of his aims for last week's Democratic nominating convention. His acceptance speech, as well as endorsements from retired generals and admirals, were aimed at establishing his bona fides as a commander in chief.

As yet, however, there is not evidence that Bush's traditional advantage on national security has been undermined in lasting ways. In mid-June -- a time when Bush was being barraged with negative news from Iraq -- the number on "handling terrorism" was even. A few weeks later, Bush was back to a nine-point lead.

Matthew Dowd, a strategist for the Bush-Cheney campaign, predicted Kerry's improvement will be similarly ephemeral. "They said 'strong, strong, strong,' for four or five days. They closed the gap some," he said. But he said the numbers keep returning to their previous averages, no matter the bounces in any given week's news, because "people see the president as more of a strong and decisive leader than John Kerry."

Bush yesterday demonstrated another advantage enjoyed by all incumbents: the ability to drive news and set policy instantly. He endorsed the key recommendations of the commission investigating the Sept. 11, 2001, attacks, including the call for a national intelligence director. Kerry's campaign said Bush was following the Democrat's lead on this. But a similar controversy two years ago -- over which party was first to endorse a Department of Homeland Security -- broke that time in favor of Bush, even though he had initially resisted the idea.

Mark Mellman, Kerry's pollster, said the campaign does not intend to stay on the defense. "We're trying to change that dynamic -- we are on offense across the board."

Peter D. Feaver, a political scientist at Duke University who studies national security and public opinion, said he believes Bush probably remains "in the driver's seat" on terrorism. If there is a terrorism incident before the election, he said, there would probably be a "rally around the flag" impact that would favor any incumbent, so long as the administration was not seen as politicizing the incident, as occurred in Spain after a pre-election terrorist attack in the spring.

A Kerry adviser, speaking on the condition of anonymity because of the sensitivity of the issue, said the campaign has discussed at length what would happen in the event of an attack and reached a conclusion similar to Feaver's, with any effect being strongest the closer to Election Day. But this adviser said campaign officials have concluded there is little that can be done to anticipate such an effect or counteract it.

Regardless of how events unfold, Kerry's strategists believe they must pass a threshold test in the public mind of being seen as capable on national security issues generally and terrorism specifically to be heard on other issues. But this does not mean the election will pivot on terrorism.

Voters who say that terrorism is the most important factor in their votes favor Bush by a margin of 83 percent to 15 percent, according to the Post-ABC poll. But most voters rank two other issues, the economy and Iraq -- over which a majority in recent Post-ABC polls are troubled by the course of events -- ahead of terrorism as being most important to them.

For the roughly one in five voters who are undecided or who register a preference but say they might change their minds, the economy is by far the most important issue -- cited by 36 percent, vs. 16 percent for terrorism.

Kerry's running mate, Sen. John Edwards, suggested in Florida that terrorism alerts of the sort the administration issued for prominent financial buildings in Washington and New York may make some voters feel that they would be unprepared if the worst happened. "I guarantee you, if we went house to house, there would be almost nobody here that knows what they're supposed to do if a terrorist attack occurred," he said. "And this is where leadership is needed -- desperately needed. What happens in the middle of the night? How are you supposed to find out when you don't have your television on?"

Staff writer Lois Romano with Edwards and assistant polling director Claudia Deane contributed to this report.

President Bush, with Homeland Security Secretary Tom Ridge and Secretary of State Colin L. Powell, discusses creating a national intelligence chief.