The hope of the civic-minded is that elections are decided by meaningful events such as presidential debates.

We like the picture of thoughtful voters gathered around their televisions, often with family and friends, thinking hard about the issues and the character of the candidates. We think of these small groups turning off their sets at the end of the encounter (or after watching a half- hour or so of spin) and discussing the momentous decision they are about to make.

There is, in fact, a good deal of truth in this picture. As Sam Popkin of the University of California at San Diego showed in his classic book "The Reasoning Voter," free elections are not random events. Voters don't pick presidents through a process as thoughtless as, say, throwing darts at a board.

"Voters actually do reason about parties, candidates and issues," Popkin wrote. "They have premises and they use those premises to make inferences from their observations of the world around them. They think about who and what political parties stand for; they think about the meaning of political endorsements. They think about what government can and should do. And the performance of governments, parties and candidates affects their assessments and preferences."

Popkin is a hardheaded scholar who reached this conclusion only after an elaborate study of how voters think and react. He does not pretend that all voters have the same amount of information. On the contrary, many voters operate on the basis of "low information rationality" -- more popularly known, Popkin notes, as "gut" reasoning. They pick up clues here and there and work those clues through their own preferences and values. Voters use "shortcuts" -- and why not? Debates are among the most efficient shortcuts.

But debates don't happen in a vacuum. Many voters who watch them have already made up their minds. They observe them with a rooting interest: Boy, did my guy land a great punch, or, more recently, why did he sigh so much?

The undecided, too, bring assumptions to the table, many of them shaped by the campaign that has gone on before. President Bush and his backers have so relentlessly used the term "flip-flop" about John Kerry that it has seeped into the consciousness of even Kerry supporters. Bush surrogates such as Vice President Cheney have been willing to state or insinuate that terrorism would be more likely on Kerry's watch. Many high-minded people have condemned this as a new form of McCarthyism. But the condemnations don't seem to matter nearly as much as the planting of an idea, a doubt, a question.

One can be certain -- I write these words before the debate -- that no matter what has transpired, Bush's spinners will interpret the debate as proving the very points they had been making before a single word was spoken.

No matter what Kerry has said about Iraq, a Bush spokesman will pronounce that the Democrat has just taken his 28th or 34th or whatever position on the war. No matter how Bush does, his spinners will describe him as resolute and consistent. Verbal gaffes, if any, will be a sign of the president's "plain-spokenness," factual errors dismissed as less important than the voters' certainty over the president's "values."

It is a sign of a successful -- if often despicable -- campaign that all this can be known in advance. Going back to last spring, the Bush campaign has been relentlessly "on message" in pursuit of a single goal: to tear down Kerry and make it impossible for the decisive sliver of undecided voters to pull a Kerry lever, punch a Kerry hole or touch his name on a screen. Bush knows that many voters aren't happy with him, so he has decided to make them even less happy with Kerry.

That it is a bit harder to pre-spin the debate for Kerry is evidence of the high hurdle the Democrat had to overcome. Kerry's job in the debate was to be relentless in forcing the election to turn on Bush's record -- and yet to look attractive, strong and even genial in the process. A twofer is harder to spin than one plain message.

So how will the reasoning voter sort through the facts and the spin? First by following her gut. And second by asking: If an incumbent has had to spend so much time tearing the other guy down, what, precisely, does that say about the incumbent's record? That happens to be the reasoning process that led, 24 years ago, to the election of Ronald Reagan over Jimmy Carter.