This fall's presidential debates will pit George W. Bush's folksy manner and big-picture brand of policymaking against John Kerry's (search) more cerebral outlook and nuanced world view. Each is a proven debater who knows, only too well, what personal pitfalls to avoid: Bush must stifle the smirk, for instance, and Kerry must cut short his rhetorical rambling.

They'll be under careful scrutiny in a trio of debates, 4 1/2 hours in all, starting Thursday in Florida. In the past, 30 million to 40 million people have tuned in. Almost one-third say the debates will be a deciding factor in their vote on Nov. 2.

On paper, Kerry would seem to have just the right resume to thwack the president in this type of setting. A high school and college debate champ with two decades of Senate repartee under his belt, Kerry knows intimately the details of policymaking and how to argue any side of an issue.

And that may be his problem. Sometimes Kerry sounds like he IS arguing every side of an issue.

His greatest weakness, in the view of political scientist Bruce Buchanan, is "getting tangled up in qualifying locutions of one sort or another. No clear, clean expository lines. Too many qualifications. Too many embellishments. Not enough editing."

"There's an innate caution to him," says Buchanan, who teaches at the University of Texas at Austin, "indicating a fear of not having covered all the bases, and that often leads him to say too much."

The president, by contrast, is rarely accused of offering too much information. He is militantly "on message," often repeating a few set points over and over.

"Bush debates the way Chris Evert plays tennis — no unforced errors," says Democrat Paul Begala, who played the part of the president in rehearsals with Al Gore  for the 2000 debates. "He doesn't get out of his game. He won't try to get into philosophy and nuance and deep thinking."

Where Bush can get into trouble is if he's forced out of his comfort zone, and becomes flustered. Or if his single-mindedness starts to look simple-minded, given the profound uncertainties surrounding Iraq , the war on terrorism and other matters, says Wayne Fields, an expert on political rhetoric at Washington University in St. Louis.

"His strongest quality is also a kind of weakness to be exploited, so you don't know how this is going to play out," said Fields. "If all of a sudden the situation looks more complicated, and Kerry is able to show he can take things on and master them, then this could turn against Bush."

Viewers will tune in as much to get a feel for the candidates' personas as for their policies. Everything from their hand gestures and mannerisms (remember Gore's heavy sighs in 2000?) to their physical appearances will be open to judgment. It all feeds into voters' decisions about whether they can relate to candidates, and trust them.

Robin Lakoff, a professor of linguistics at the University of California at Berkeley, said that when she turns down the volume and just watches Bush and Kerry, "it clarifies why Bush is more effective. He has the nonverbal stuff, the facial expressions and gestures." He furrows his brows, he seems to look through the camera to make eye contact, she says.

Kerry, by contrast, "really has no facial expression," says Lakoff. "He just talks. ... I think Kerry's long sentences and lack of intonation and facial expression say, 'Yes, I'm very smart but I'm kind of phoning it in.'"

Jurgen Streeck, a communications professor at the University of Texas at Austin, said that while Kerry is not a very lively communicator, the debates may provide a good setting to showcase him as "a thoughtful speaker."

Bush, meanwhile, must guard against smugness.

"He has that kind of smirk," says John Fritch, head of the communications department at the University of Northern Iowa and director of the National Debate Tournament. "Given the issues that we're dealing with, the casualties in Iraq, an inappropriate smile will not go over well."

Says Begala, "If I were prepping Bush, I would warn him about crossing the line from self-confident to cocky. People like his self-confidence but there are moments, particularly when he's jacked up on adrenaline, when he crosses that line."