Democratic presidential challenger John F. Kerry begins the fall campaign on the defensive and faces the urgent necessity to make the race about the economy and other domestic concerns, after President Bush used his convention to say this election is all about keeping the country safe.

Bush rode out of New York after a four-day convention tightly focused on terrorism -- and what Republicans say is the striking contrast between the credentials of the two candidates for commander in chief. Democratic strategists now say it is essential for Kerry to seize back the initiative that was lost during August.

"The most important thing is for Kerry to reengage the economic debate and . . . to be crystal-clear in framing the choice for voters," said Geoffrey Garin, a pollster whose clients include the Democratic National Committee. "The Republicans were very disciplined in how they wanted to frame the choice. . . . I think this is the moment where Kerry has to lay that out clearly."

Kerry strategist Tad Devine said the campaign is moving to shift the debate from terrorism to health care, jobs and other domestic issues under the theme that "a stronger America begins at home."

"They want to move the electorate toward Bush's perceived strength," he said. "We're trying to meet the voters where they are."

A senior Kerry adviser agreed. "They made a determination that they're going to run strictly on terrorism and commander in chief," he said of the Bush campaign. "We need the voters to make a different decision."

On the GOP convention's final day, Kerry strategists told reporters in New York that they were launching a television ad campaign to emphasize the economic issues they believe are uppermost for voters in battleground states. They plan to portray Kerry as the candidate who cares most about local concerns, whether it is nuclear waste storage in Nevada or job loss in Ohio.

The first post-convention indicators show that Kerry will be running from behind. A Newsweek magazine poll released yesterday showed Bush leading Kerry 52 percent to 41 percent, with independent Ralph Nader at 3 percent. A Time magazine poll released Friday showed a similar 11-point margin, although two other polls taken during the convention showed the race statistically tied.

The two sides are operating under different theories of how to win this election -- and not only whether terrorism or the economy will tip the balance in November.

"What we [the Kerry campaign] have to do is look at all these different places, these individual battlegrounds, state by state, and figure out how we're going to win each and every one of them," Devine said. ". . . I think they have a national campaign and a national message, which isn't going to differ a lot if you're in Florida or Ohio or Nevada."

Bush's side does not disagree. Matthew Dowd, his chief strategist, said that over the next two months, the vote in the battleground states will follow the national trends, rather than deviate significantly because of local factors. "I don't think there's anything going on in any individual state," Dowd said. "National and individual battlegrounds are adjusting the same."

Despite their differences on how to communicate with voters, the two campaigns agree on the importance of state-by-state, even county-by-county, voter turnout strategies. Here the GOP expressed concerns about the strength of the Democratic ground game, which has been augmented by independent organizations funded with large, unregulated "soft money" donations. Republicans are counting on predominantly volunteer organizations to register and turn out voters and believe that they have built on their successful model from the 2002 midterm election.

The two campaigns have been advertising in about 20 states, but the real battles are concentrated in far fewer, Kerry and Bush strategists say.

Democrats are competing in Colorado, North Carolina, Louisiana, Arkansas, Arizona and Virginia, but Bush appears to have the upper hand in all of them. Republicans are doing the same in Oregon, Washington and Michigan, but face uphill battles.

Democratic states in 2000 where Republicans believe they have a serious chance of winning include Iowa, Minnesota, New Mexico, Pennsylvania and Wisconsin. The states Bush won in 2000 that Kerry strategists hope to deny him include Florida, Nevada, New Hampshire, Ohio, West Virginia and perhaps Missouri.

Conventional wisdom is that a presidential race is 51 separate elections -- one in each state and the District of Columbia. Democrats have not forgotten that in 2000, Al Gore piled up a half-million more popular votes than Bush by rolling up large margins in states such as California and New York, but lost in the electoral college because of narrow defeats in Florida, New Hampshire and a few other states.

Bush is targeting a national audience of undecided voters, largely with a single message -- that he is a better bet to keep the country safe. Dowd and other Republicans argue that because the number of persuadable voters is small and they are widely dispersed, a national message is the best way to reach them.

Dowd said most of the undecided voters are in suburban precincts, most are white and they tend to be older. There are roughly equal numbers of men and women, and more of the undecideds are churchgoers than in the partisan ranks, he said. Traditionally, undecided voters split heavily against an incumbent president. But Dowd said that, compared with April, when the undecided pool appeared tilted in Kerry's direction, they now are as likely to support Bush as the Massachusetts senator.

Reaching those voters -- who make up less than 10 percent of the electorate -- will be a priority for both campaigns, along with intensive efforts to turn out their own partisans. Dowd said that unlike the partisans, the undecided voters pay attention largely to big events in the campaign, such as the conventions, and that the upcoming debates will play a crucial role in their decision-making.

Polling shows that jobs and the economy are the No. 1 concern of voters, and they have been the staple of Kerry's campaign all year. Democrats contend that overall, incomes have declined since Bush took office, and they say that the tax cuts at the heart of his economic policy have increased inequality and left millions of workers and their families struggling to pay bills.

But Bush now has economic figures to bolster his claim that the country has gone through the worst of the downturn and is coming back, and pre-convention polls showed voters rated Bush equal with Kerry on managing the economy.

On national security issues, Bush maintains a strong advantage. Convention speakers -- including Bush and Vice President Cheney -- avoided mention of "weapons of mass destruction" and other failures of prewar intelligence in Iraq, while the president offered a hopeful scenario for the development of democracy in that country and an eventual shift of security responsibilities to Iraqi forces.

Although polls show increasing majorities of Americans questioning the wisdom of going to war with Iraq, Kerry is inhibited on that issue by his history -- voting for the original use-of-force resolution, then against the bill for funds for operations and reconstruction in Iraq.

Bush regularly draws cheers on the road -- as he did in Madison Square Garden -- when he says that in the face of Saddam Hussein's defiance, he had to ask himself: "Do I forget the lessons of September 11th and take the word of a madman, or do I take action to defend our country? Faced with that choice, I will defend America every time."

By contrast, Kerry's promise to enlist help in Iraq from other countries, thus speeding the day when U.S. forces can withdraw, rarely evokes cheers. Former representative Vin Weber (Minn.), an adviser to the Bush campaign, said the reason the response seems weak is that Kerry's position on Iraq over the past 13 years -- going back to his vote against the first Persian Gulf War -- is "simply incoherent."

As for the threat of another terrorist attack on the United States, polls suggest that it is less of a day-to-day worry to voters than Iraq or the economy -- even though most say they expect the terrorists to strike somewhere. But polls do not measure the emotional pull of the Sept. 11, 2001, assault and the way the nation rallied in response.

When Bush speaks of that period -- the defining moment of his presidency -- he is trying to recapture the support and admiration he had at the time and combat the dissatisfaction over his Iraq policies. "They're trying to bring people back to where they were on September 11 and away from the feeling now that the country is moving in the wrong direction," Kerry pollster Mark Mellman said.

Kerry used most of the Democratic convention to make the case that he would be an equally strong and vigilant protector of the nation -- surrounding himself with retired generals and admirals and fellow Vietnam War veterans, and dwelling at length on his record as a wounded and decorated hero of that struggle.

In August, his campaign was thrown badly off balance by attacks on that service record from a group of disaffected fellow veterans, some with close ties to the Bush campaign, which tried to distance itself from the assault but did not denounce it.

Mellman told reporters last week that, despite the August turbulence over ads by the Swift Boat Veterans for Truth, Kerry has passed the threshold of acceptability to be commander in chief. "We don't expect to beat Bush on national security . . . and I think all the evidence suggests that John Kerry has met and exceeded that threshold for a majority of the American people," he said.

Democratic strategists inside and outside the campaign argue that Bush has left Kerry an opening to make the final two months of the election a debate about domestic issues. "We can talk to people about things they care deeply about," Devine said, pointing to the cost and availability of health care, the expense of college and the uncertain jobs picture. "Bush does not have enough credibility to talk directly to people about things like that."

Devine said the Kerry campaign will place ads around Bush appearances in battleground media markets, reminding voters of what Bush said in 2000 and what has happened since.

The two agendas are likely to collide when Kerry and Bush meet for their first debate, tentatively scheduled for Sept. 30 in Miami. The subject of that debate will be negotiated by former secretary of state James A. Baker III for Bush and lawyer Vernon E. Jordan Jr. for Kerry when they sit down on behalf of the two campaigns. Each is likely to push to put his candidate's favored topic at the forefront.

Democratic presidential nominee John F. Kerry, shown campaigning in Akron, Ohio, faces the task of getting undecided voters in his camp before Nov. 2.