For Sen. John F. Kerry (D-Mass.), stuck in second place with just five weeks left in the presidential campaign, the debates that begin this week may be the best chance remaining to close the sale with voters and beat President Bush.

Widespread polling, stubbornly consistent for months, finds Bush vulnerable. Voters report that they are unhappy with the war in Iraq, the state of the economy and the general direction of the country. Yet the same polls indicate that more voters do not like his Democratic challenger than do.

Beating Bush in these debates -- the first on Thursday at the University of Miami -- will be no easy matter, judging from the extensive record Bush and Kerry have compiled in televised face-offs. The president is an unorthodox debater but an effective one, especially against candidates schooled in the traditional rules of debate, such as Kerry.

"Both of them are good. Bush has never lost a debate that I know of," said Democratic campaign strategist Chuck Dolan. "He's very cool; he stays on message. He approaches debates in a very different way than most other politicians."

Against that, Kerry brings an aggressive style and a command of the minute details of policy -- qualities, it must be said, that were not enough to win when Texas Gov. Ann Richards and Vice President Al Gore took them into battle against Bush in 1994 and 2000.

The first encounter will focus on foreign policy. Bush's debate history shows one thing above all: His themes in debate are the same ones he preaches on the trail. By that yardstick, Bush will defend the war as a tough decision with no acceptable alternative. "Do I trust Saddam Hussein? Do I forget the lessons of September the 11th or take action to defend this country?" Bush repeats in speeches across the country. "Given that choice, I will defend America every time."

Kerry's tone on the campaign trail in recent days suggests that he will go after the president aggressively on Iraq. "The invasion of Iraq was a profound diversion from the battle against our greatest enemy: al Qaeda," Kerry said last week. "George Bush made Saddam Hussein the priority. I would have made Osama bin Laden the priority."

But while civics teachers, activists and other concerned citizens might not like to hear it, presidential debates are at least as much about style as substance, veterans of the process agree.

"They call them debates, but they're not debates," said Republican debate coach Sheila Tate. "They're platforms for the two candidates to position themselves" for an audience of undecided voters who, research shows, tend to rely on their gut, Tate said.

Elaborate rules, negotiated by the rival campaigns, guarantee that the sessions are more side by side than head to head; direct engagement is limited. What matters most is not which candidate dominates the room but which one comes across most powerfully, in planned and unplanned stylistic ways, on television. And as the late, legendary BBC interviewer Robin Day once explained, television "strikes at the emotions rather than the intellect."

Dolan learned this lesson the hard way in 1988. He was in the room when the Democratic nominee, Gov. Michael S. Dukakis of Massachusetts, fielded a question about his opposition to the death penalty for murderers: Would he feel the same way if it were his own wife who was slain?

"We could see in the hall that it didn't go well," Dolan said of his candidate's coldly detached answer. "Then I saw it later replayed on television, and you could see it was devastating."

Many Democrats who watched an often stumble-tongued Bush go from inexperienced candidate for Texas governor to incumbent president battling for reelection have wondered where he gets his ability to win debates against candidates with more seasoning, more verbal dexterity, and a greater supply of names and numbers at their mental fingertips.

Part of it, Tate argues, is that Bush "knows his mind" and thus projects himself clearly. But it is also true that, for a man who is not a trained actor, Bush has a highly expressive face on television, and he uses it to good effect in underlining his points while undermining his opponent.

True, many people recoil at his famous smirk. But Bush knows how to signal incredulity with a slightly cocked head and a flicker of eyebrow-arch. His more emphatic brows-raised, lips-puckered look says: "Get a load of that blowhard." He has a half-wink that signals he is about to land a punch and a half-squint that says, "I really, really mean what I'm saying now."

Among the most effective moments in Bush's debate career came in his third and last encounter with Gore in 2000, and it stemmed not from verbal prowess but from acting skills.

The format that night allowed the candidates to roam about the stage. Gore was trying to bear down on a mushy answer that Bush was giving on a controversial topic. As if to underline his strength and Bush's weakness, Gore edged closer and closer to his opponent until he was looming behind him like the monster on the late, late show. Bush turned slightly, did a quick double take, and reduced the audience to laughter.

Deflated, Gore returned to his stool.

Bush matches these signals to a speaking style no professional debate coach would ever approve. Judging from the evidence, it works for him. According to friends and advisers, Bush has a West Texas conviction that people who talk fancy are not to be trusted. So, they say, he tries to project steadfastness and sincerity by using short, simple declarative sentences and a lot of one- and two-syllable words.

In his first televised debate, in which Bush challenged the well-spoken Richards, the novice was cool as a creek while dodging her attacks on his inexperience. But his strongest moment may have come when he was asked about casino gambling.

"I'm opposed to casino gambling," he answered. And shut up.

Richards spoke next. She said she, too, was against casinos, but then went on at length to explain all the exigencies of law and policy that might someday force her to change her mind. When she finally finished, the moderators asked Bush if he wanted to expand his answer.

Flashing his "what a blowhard" look, Bush declined. "I oppose casino gambling," he repeated.

It so happens that one thing Kerry and Bush have in common is the classic comedy "Animal House." Kerry has said it is his favorite movie, while Bush was president of the fraternity that has been called Yale's nearest approximation of the mythical miscreants of Faber College.

The film holds a clue both to Bush's debate skills and to his likely strategy for engaging Kerry on the unspoken, emotional level of television. A toga-clad John Belushi enters the frame. The audience already knows that he's a fairly inarticulate character with a loose grip on the facts. Halfway down a flight of stairs he encounters a long-faced, very earnest fellow strumming a guitar and singing to a group of young women who might well be Romance language majors.

The camera homes in, devastatingly, on Belushi's contemptuously arched eyebrow.

You don't have to guess which candidate is which here. The guitar-playing Kerry, who once bragged to Vogue magazine that his chocolate chip cookies are special because he uses imported Swiss chocolate, and the frat house wiseguy Bush.

But that's not the persona Kerry displayed in 1996, during his marathon of televised debates against Gov. William F. Weld of Massachusetts. That Kerry was a puncher, not a poseur. Kerry is a classically trained debater, having learned the ropes on debate teams at prep school and at Yale. Kerry is quick on his feet, he pushes hard and he can be ruthless.

All this was on display in the knock-down, drag-out sessions with Weld, a serial drama of shouted statistics, rapier gibes and cheap shots landed so quickly no referee could flag them. Here's the flavor of it, from a debate in which Weld was trying to defend his record on education while Kerry kept butting in.

Weld: We are funding K-12 education . . .

Kerry: Forty-nine states . . .

Weld: A billion dollars more than we were in 1993. I don't know how the priorities could be any clearer than that. . . .

Kerry: . . . That's the problem, Governor. You don't understand how they could be greater. And if you did understand, you'd have a different set of priorities!

A wave of applause ended the exchange before Weld could point out that he said "clearer," not "greater."

Like Bush, Kerry is disciplined about returning to his key themes over and over again. Against Weld he was relentless about tying the moderate governor to the far more conservative Republican leaders in Washington. This, plus President Bill Clinton's landslide victory in Massachusetts, allowed Kerry to stave off Weld's challenge.

The lesson that Richards and Gore both learned, however, is that debate skills alone are not enough to shake Bush. Kerry must latch on to something more. And he may find clues in the two debates in which Bush has not done well. They have something in common: Bush was not playing the role of challenger.

In 1998, as the incumbent governor of Texas, Bush met Democrat Garry Mauro in a single debate. He was uncharacteristically "nervous and defensive," in the view of Austin political reporter Dave McNeely. In 2000, Bush's weakest performance came in a primary season face-off against surging underdog Sen. John McCain (R-Ariz.).

"Bush seems to be more comfortable portraying himself as the outsider, but he can't do that anymore," said Bill Benoit, a professor of communication at the University of Missouri who has extensively studied presidential debates. As the outsider, Bush has freedom to stay on the broad, thematic level, above the thicket of details.

"He used the idea that he was on the outside to say, 'Don't pigeonhole me,' " said Benoit. As the outsider, Bush has been able to strike appealing chords -- a promise to be "different" from other leaders, a promise to be "a compassionate conservative," a promise to be "a uniter, not a divider" -- while allowing viewers to convert those chords into music of their own liking.

That won't be so easy this time. "Who is more insider than the president of the United States?" Benoit asked.

Some critics charge that presidential debates, as structured, aren't much good for determining anything beyond who best survives the strange test of 90 minutes under a relentless gaze. A gesture as small as President George H.W. Bush's glance at his watch in 1992 can be enormously damaging. Many of the most important issues a president will face never come up for discussion -- Kosovo came in for far more discussion in 2000 than al Qaeda -- and other issues, such as Social Security, are eternally talked about but never resolved.

Nor are the debates necessarily a good barometer of brains or character.

When he appeared in the 1992 vice-presidential debate, retired Adm. James B. Stockdale was: a former college president, a senior fellow at the Hoover Institution, an authority on the Stoic philosopher Epictetus (whose work he read in the original Greek), the holder of 11 honorary doctorates and the only man in naval history to wear three stars, aviator's wings and the Medal of Honor.

All that evaporated under the harsh stare of the camera when, with earnestly wide eyes, Stockdale introduced himself by posing two rhetorical questions: "Who am I?" and "Why am I here?" The hero-scholar came across like an addled old man.

Imperfect, even misleading, the debates are nonetheless the crucial final act of this close-fought and momentous campaign. Kerry must do what he has not been able to do so far: make himself a more plausible president for the last few undecided voters than the one they already have.

"In the final analysis, there's a very small percentage of people who haven't made up their minds by now," Tate said. "And they make their decision based on who they like, who they think they can trust -- and it's a very amorphous, very emotional decision."

Political researcher Brian Faler contributed to this report.

Sen. John F. Kerry (D-Mass.), left, and President Bush are expected to bring different strengths and styles to their three televised debates, the first of which will be held Thursday. Kerry, a former high school and college debate star, is aggressive and well-versed in the facts. Bush often connects with listeners on an emotional level.