President Bush headed Monday toward his first face-off with Sen. John F. Kerry by mocking his challenger as a vacillator who could spend the whole time debating himself, while Kerry attacked Bush on issues from Iraq and the economy to dairy policy.

The candidates' appearances in two of the election's marquee battlegrounds -- Bush in Ohio and Kerry in Wisconsin -- broke with the tradition of pre-debate seclusion, and the events reflected the moods and planned themes of each camp. The president, riding a wave of encouraging polls, plans to use plain talk and dashes of humor to call attention to what he considers shifts and gaps in Kerry's record, according to aides. The senator from Massachusetts, slipping with crucial Democratic voting blocs, plans to continue fighting back with a newly focused and unsparing critique of the president's performance as commander in chief, particularly on Iraq, his aides said.

At dusk, Bush staged the largest rally of his campaign, arriving at a historic park outside Cincinnati in a star-spangled bus, with cranes suspending a huge flag and a giant map of the Buckeye State over a vast, roaring crowd. While some in Bush's inner circle warned against overconfidence, the event planners replaced the usual country-music finale with "Celebration," and the overhead speakers blared tunes from "Top Gun."

Flag-waving crowds of hundreds turned out in small towns to wave at Bush's cavalcade at it wended through Republican territory in southwest Ohio. Tieless and with his sleeves rolled up, Bush joked at the rally in West Chester that it had been "a little tough to prepare for the debate" because Kerry "keeps changing his positions, especially on the war."

"I think he can spend 90 minutes debating himself," Bush said, as the crowd chanted, "Flip, flop, flip, flop." Turning serious, Bush said, "You cannot lead if you don't know where you stand. I'm going to continue to speak as clearly as I can and tell the people what I believe. And I'm not going to change positions when times get tough."

Rep. John A. Boehner (R-Ohio) said as he introduced Bush that it was "the largest political event ever held in Ohio," and a Secret Service count put the crowd at 41,000. The Bush campaign said its previous records were 22,000 for an event in Ohio and 23,000 for one in Missouri.

At a much smaller event, Kerry, dressed casually but with a cutting message, appeared determined to cover a long laundry list of his positions during a 70-minute town meeting in rural Wisconsin, near the resort where he has pitched his debate camp. Kerry seemed to be talking as much to swing voters in the battleground state as he was trying out lines of offense and defense for the debate on Thursday.

"Are you telling me seriously that people in Wisconsin are going to return to the presidency a man who promised jobs and lost them?" Kerry asked. "I think the good common-sense, fiscally responsible, conservative citizens of Wisconsin know that it's our responsibility to pay our bills and not dump them on our kids."

Kerry told his audience of nearly 300 in the Dairy State that Bush had a secret plan to implement after the election that would hurt milk producers, and accused the president of opposing an effort to extend price supports for loss of milk income when prices drop.

"As a senator representing Massachusetts, I fought for the dairy compact and fought to have our dairy farmers get help," the four-term lawmaker said. "I'm running for president of the United States now, and I intend to represent all the farmers of America." Bush-Cheney spokesman Brian Jones said the accusation about Bush had no merit.

Kerry told the town hall that Bush is "still trying to hide from the American people what needs to be done in order to be successful in Iraq," and called it "questionable" whether there can be democratic elections in Iraq by January because of the turmoil in the country.

Kerry said he recently told a voter that it is all right to switch horses midstream when the horse is drowning. It was the job of Kerry's running mate to remind voters in another battleground state that the situation in Iraq is a dangerous distraction from the war on terrorism. John Edwards called on Bush to answer why his administration has not found Osama bin Laden.

"Where is Osama bin Laden?" a stern-faced Edwards asked, jabbing the air with his finger as he addressed a vocal crowd at Victory Park in Manchester, N.H. "This is a serious issue. This man masterminded the murder of 3,000 Americans."

With both candidates planning to spend much of Tuesday preparing for the first debate, officials of the Commission on Presidential Debates said the Bush-Cheney campaign is leaving open the possibility of pulling out of the second or third debate because the commission has decided not to sign the debate agreement between the two campaigns. The nonpartisan commission said the campaigns' request for such a signature was a first since the nonpartisan group was formed in 1987. Kerry's campaign said he will debate in any case, but the Bush campaign has sought assurance that all of the restrictions on the format will be enforced.

The commission tried to pacify the Bush campaign by issuing a statement saying it would abide by the agreement. But the group did not want to become a formal party to it because of the possible effect on its tax-exempt status. The four moderators also have declined to sign the agreement, the commission said.

The statement specifies that "there will be no departure from the terms of the memorandum without prior consultation with and approval by the appropriate campaign representatives." Bush-Cheney deputy campaign manager Mark Wallace said he is "pleased that the commission has agreed to uphold and enforce" the agreement.

Romano reported from Spring Green, Wis. Staff writer Matthew Mosk, with Edwards, contributed to this report.