I understand why it was important for Obama to show some backbone tonight after the intense Clinton assault of late. And, if that was the goal, he clearly accomplished it. Having said that, I suspect the Clintons would be delighted to turn this race into an endless game of tit-for-tat.

With the possible exception of health care--about which more later--it's not that Hillary dominated any particular exchange. In fact, I think Obama won more than his fair share. But the whole tenor of the first half of the debate was too small-bore and sharp-edged for Obama's good. He'd mention Hillary's dubious bankruptcy bill vote, then she'd come back with a dubious Obama vote against capping credit-card interest rates. He'd hit her for working as a corporate lawyer while he was fighting against the Reagan revolution; she'd come back with his work on behalf of slum-lord Tony Rezko. I don't see how Obama wins the nomination if voters can throw up their hands and dismiss them both as typical pols. The first part of tonight's debate made that a little easier.

At times, Obama even seemed to acknowledge this himself. He'd try to step back and go thematic--as when he explained that "the larger reason that I think this debate is important is because we do have to trust our leaders ... [I]f we can't, then we're not going to be able to mobilize the American people." But it was too late. It's hard to revert to Marquis of Queensbury rules while you and your opponent have each other in a headlock.

As for health care, this is the one portion of the debate that benefited Hillary on its own terms. Politically and rhetorically, the beauty of Obama's anti-mandate position is that it sets up the (rather compelling) argument that people don't have health insurance because they can't afford it, not because they don't want it. Problem is, the claim is debateable on the merits, and tonight Obama opened himself up to an extensive debate about it. Worse, it was one that invited a double-team, as both Hillary and John Edwards take the opposite view.

During what followed, Edwards got to make a compelling, if slightly overwrought, rhetorical point of his own--that allowing people to opt out of the healthcare system is akin to George W. Bush letting people opt out of Social Security, thereby undermining the system. (I say overwrought because Obama's trying to bring many more people into the healthcare system while Bush wanted to do the opposite for Social Security.) And both candidates pounced on Obama's suggestion that people could be fined for not having insurance even when they couldn't afford it. (Clinton and Edwards insisted there would be subisidies to make insurance affordable. Who's right here basically depends on your definition of "affordable"--though there would clearly be some people who, for whatever reason, have to strain even with the subsidies. Jonathan Cohn wrote more about this here.)

Finally, I thought Hillary had her best moment of the night during the healthcare back-and-forth. "I think that the whole idea of universal health care is such a
core Democratic principle that I am willing to go to the mat for it," she said. "I am not giving in; I am not giving up; and I'm not going to start out leaving 15 million
Americans out of health care." Later she added, "When you come up with a universal health care plan and you don't have any wiggle room left, you know that you're going to draw a lot of political heat. I am not running for president to put Band-Aids on our problems." The effect was to distill the entire case for Hillary into a single response. She was passionate, courageous, tough, and exceedingly well-informed--everything that would make you proud to have her as a nominee.

That said, the second half of the debate evoked similar feelings for Obama. His response to a question about Bill Clinton being the first black president was incredibly winning--thoughtful and sophisticated in spots, completely endearing in others. (E.g., "I would have to investigate Bill's dancing abilities, and some of that other stuff, more before I could accurately judge whether or not he's a brother.") I also thought he reached his usual rhetorical heights with a response about redrawing the political map, in which he worked in a riff about why he's running for president. "I believe that I can inspire new people to get involved in the process," Obama said. "[T]hat I can reach out to independents and, yes, some Republicans who have also lost trust in their government and want to see something new."

The question I have after tonight is: Which half of the debate is likely to define the rest of the campaign? If it's the first half, that could mean trouble for Obama. If, on the other hand, the first half was just a one-off demonstration of mettle--and maybe a warning shot to the Clintons--I think it was probably helpful, even necessary. Moreover, if the second half was an indication that Obama can easily revert to his seductive high-mindedness even after sticking a shiv in Hillary's gut, that could bode pretty well for him. We shall see.

--Noam Scheiber