"Strength builds peace. Weakness invites those who do us harm," the ad says, a suggestion that Kerry would be a weak leader in wartime and a country headed by him would be vulnerable to terrorist attacks.
The ad accuses Kerry of "refusing to support our troops in combat" and trying to severely slash intelligence budgets and eliminate military weapons after the first attack on the World Trade Center.
The two candidates debate foreign policy at the University of Miami on Thursday night, the first of three scheduled debates. Both candidates were spending Tuesday preparing for the meeting.
Responding to the new ad, Kerry's campaign accused the Bush campaign of "using the politics of fear" to mask the president's "mistakes in Iraq."
"They don't want people to know that President Bush has no plan for Iraq, or that his failures there have made it harder to fight the war on terror," said Chad Clanton, a Kerry campaign spokesman.
The Kerry campaign is expected to release a new ad Wednesday using President Bush's now infamous "Mission Accomplished" speech declaring an end to major combat in Iraq.
Kerry was continuing his debate preparations Tuesday in southern Wisconsin.
On Monday, he held a town meeting in Spring Green, Wis., not far from the golf resort where he's boning up for Thursday night's face-off.
Reaching out to Wisconsin voters, Kerry said Mr. Bush has created only 200 jobs in a state that has lost 67,000 manufacturing jobs on his watch.
"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 commonsense, fiscally responsible, conservative citizens of Wisconsin know that it's our responsibility to pay our bills and not dump them on our kids and on future generations."
Mr. Bush was preparing for the debate Tuesday at his ranch in Crawford, Texas. White House communications director Dan Bartlett said the president is done sparring with mock-debate partner Sen. Judd Gregg, R-N.H.,
Mr. Bush planned a low-profile day at the ranch, "crystallizing" his thoughts on policy and sharpening zingers, Bartlett said.
While top White House staff tried to lower expectations about the president's debate performance, not so the first lady. In a radio interview with CBS News, Laura Bush said her husband will do very well.
"He has great characteristics. He says what he thinks," said the first lady. "He's very straightforward. He means what he says. I think people will see that again in this debate."
She said her husband is often underestimated. "I think that's just a fact of life. It's been a fact of life, his political life for sure."
Mrs. Bush described her husband as "pretty relaxed" about Thursday's first verbal showdown with Kerry. "I think he's looking forward to them," she said.