On Friday morning Democrats all over the United States emerged from their homes with a new spring in their steps. After the presidential debate the night before, many of them had a new experience: It was possible to be for John Kerry and not just against President Bush.

It is hard to overestimate how important Kerry's strong debate performance was for his campaign. For weeks rank-and-file Democrats had spent much energy whining and mourning. They wondered why Kerry was failing, why Republicans seemed to run better campaigns. If Kerry had bombed, the campaign was over.

Not only did Kerry avoid disaster. He finally managed to look like a leader. He spoke in short sentences, ridding his speech of a past pluperfect subjunctive tense that was all his own. He took the fight to Bush hard. But Kerry's more-in-sorrow-than-anger tone kept him from looking obnoxious or arrogant. When Bush was gracious to Kerry about his family, Kerry was gracious back. To score equally with Bush during a likability moment was a big deal.

And after being subjected to who knows how many hundreds of attacks about flip-flopping, Kerry finally managed to make the counterargument against Bush that fits the public's perceptions of the president.

"It's one thing to be certain, but you can be certain and be wrong," Kerry said. "It's another thing to be certain and be right, or to be certain and be moving in the right direction, or be certain about a principle and then learn new facts and put them to use in order to change and get your policy right." Bush's core claim that he sticks to his guns is the flip side of one of his core weaknesses: that once he decides something, he never, ever rethinks or challenges himself, no matter what the evidence says.

But the debate did more than make Democrats feel better. It demonstrated just how vulnerable Bush is. There were many moments when a fluent and calm Kerry looked more like a president than the president.

Bush is a gifted and disciplined stump speaker who can stir and amuse his carefully screened crowds and produce sharp, clean sound bites that are a producer's dream. But the Bush of Thursday night looked nothing like the Bush of the campaign trail. Ill at ease and often halting, he turned in one of the worst public performances of his presidency.

At times, he looked like he was ransacking his mind for stray facts. He kept leaning on his stump rhetoric even when it seemed inappropriate. A couple of times, he seemed to be hoping that time would run out because he had run out of things to say.

And the debate revealed the hollow core of the president's one-note campaign: The argument Bush really cared about pushing -- over and over and over -- was that Kerry sent "mixed messages" and that, as the president told Kerry, "you keep changing your positions on this war." Bush appeared obsessed with reminding people that Kerry had called the Iraq conflict "the wrong war at the wrong place at the wrong time," a phrase he repeated at least seven times.

Bush clearly hopes that the flip-flop argument and his increasingly unreal claims that all is well in Iraq will be enough to allow him to hang on through Election Day. He's assuming that no one will ask hard questions about the narrative he's weaving.

But Kerry did, and the narrative began unraveling. That was the other striking and disturbing aspect of the debate: Bush fares very badly when he is forcefully challenged. It makes you worry about his strength in circumstances he does not completely control. Since Sept. 11, 2001, the president has received a remarkably free ride. He rarely faces the media. He speaks only to partisan crowds; critics risk arrest if they show up. There is little evidence that Bush is challenged by his staff or his Cabinet. He is most comfortable when he sticks to talking points.

But suddenly, when Bush was confronted for 90 minutes by an opponent willing to go straight at him, he fumbled, he hesitated and he scowled. The Bush Scowl is destined take its place with the Gore Sigh and the Dean Scream.

The Bush forces will no doubt try to find some way to spin this debate into a Bush victory. I could be wrong but, honestly, I think that may be beyond the talents of even Karl Rove. That's why Democrats, finally, are smiling.