The last two standing after 17 debates, Hillary Clinton and Barack Obama treated each other graciously Thursday evening in Los Angeles—even as they pointed out their differences. With the Republican race taking shape around John McCain, Clinton and Obama can't afford to look like two kids squabbling in the back seat while the elder statesman drives toward the nomination.

When they differed, it was cordial and substantive. He is more of a big-picture guy, a sociologist explaining the complexities of a problem, while she offers solutions and plans and proposals. They went over familiar ground to debate junkies—her health-care mandate; his belief that people will buy insurance if it's affordable; her wariness about talking to dictators; his willingness to engage with America's enemies.

The sharpest exchange occurred over Iraq, her experience versus his judgment in opposing the war from the start. Obama won that one, co-opting Clinton's trademark slogan about her readiness to be president when he said, "It's important to be right on day one." Clinton rattled on defending her vote to give President Bush authority to wage war, which she claimed she didn't think he'd really use, until CNN moderator Wolf Blitzer asked, "Are you saying you were naïve in trusting President Bush?"

"No, that's not what I was saying, but good try," Clinton replied with a good-natured smile.

The tone of the evening was so different from last week's slugfest that it seemed only natural when a questioner in the online audience asked about the Democrats' "dream ticket," Clinton-Obama or Obama-Clinton. Obama went first, parrying the question, but concluding that "Hillary would be on anybody's short list." Hillary laughed heartily as she agreed with her rival. The good feeling may not last, but the debate framed the choice confronting Democrats as a difficult one of choosing between a candidate well versed in the byways of Washington policymaking and another with a rare ability to touch the heartstrings of politics.

Hillary did a lot to repair the ill feelings generated by her husband. In campaigning for the South Carolina primary, Bill Clinton's goal was to make it harder for Democrats to vote against his wife. Instead he made it easier, even easy for some disillusioned liberals. An Al Gore person told me, "Now you understand why Al Gore didn't use Bill Clinton in 2000." The bottom line: with the former president it's all about him, and you can't trust him to stay on message. Democrats still love him, but they've had their fill. Asked about her husband at the debate, Hillary repeated her defense of him as a passionate spouse but said this is her campaign and the presidency is a lonely job.

We don't know yet whether the South Carolina results and the campaigning before the vote was a singular event or the unraveling of Hillary Clinton's candidacy. Obama's coalition of nearly 80 percent of the black vote together with almost one quarter of the white vote allowed him to reclaim the mantle of a unifier. Bill Clinton did away with the fiction that this is Hillary's campaign. It's clear her election is wrapped up in his legacy, and he could cost her the nomination. If she does grind out the delegates and win her party's nod, he's handed the Republicans a potent line of attack for the fall campaign.