Barack Obama had one thing going for him coming into this debate: he couldn't be any worse than the last time. Mitt Romney had one drawback: he apparently couldn't be any better.

As it turned out Obama was much better. Clearer, sharper, more decisive and passionate, he challenged Mitt Romney on the facts and rhetorically he overwhelmed him. It was a rout every bit as conclusive as the first debate. Only this time the victor was Obama. Last time he barely showed up; tonight he showed Romney up.

Self-assured without being too cocky, focused without being too wonkish, he managed to strike the right balance between being firm with Romney and empathetic with the questioners. There was little of substance that he said that was different; but there was a great deal of difference in the way that he said it.

To him were gifted the best lines of the night. When taking on his Republican challenger over his shift from supporting a ban on assault weapons as governor of Massachusetts to opposing them as a presidential candidate, he said: "He was for an assault weapons ban before he was against it."

When Romney pushed him on investments in his pension funds, asking if he'd seen his pension recently, Obama responded: "I don't look at my pension. It's not as big as yours so it doesn't take as long."

When Romney was corrected by the moderator, Candy Crowley, after suggesting that Obama did not call the attack on the US embassy in Benghazi a "terrorist attack" Obama shouted with a smile: "Say it louder, Candy."

Romney did put up a fight. Approaching the debate with the same style as he did in Denver he brought his best self. Probably his best line came in an answer about why, given the hard times of Obama's first term, he should be trusted with a second. "We just can't afford another four years like the last four years," said Romney.

It is difficult to equate the stiff and impersonal figure of the conventions with the man who showed up tonight. But it simply wasn't enough. Indeed, if anything it was too much. For in his effort to reassert the control he enjoyed during the first debate he overreached.

Where he once appeared animated he now came off as aggressive. His interruptions looked desperate and his interjections were shrill. In the two weeks since his triumph in Denver he went from persuasive to petulant.

The other loser tonight was US politics. Two men circling, talking over each other, drawing on different facts and calling each other liars looked like a metaphor for much that has gone wrong in American political culture over the last generation. Town hall meetings are supposed to be less confrontational. But more caustic than consensual, this was a bad-tempered affair.

Obama's performance will energise his base and shore up the doubts of those shaken by his earlier drubbing. It will staunch the bleeding of support towards Romney but it is unlikely to reverse the flow.

They called it a town-hall meeting. But in truth there are very few towns like it. It was a room full of undecided voters: the nation is not.