The first Democratic presidential debate had one casualty: a plausible reason why Vice President Joe Biden should enter the race.

That's because Hillary Clinton deftly argued that President Obama's decision to hire her as secretary of state was all that Democrats needed to know about her judgment, and that it should put to rest questions about her vote in favor of the war on Iraq and her late embrace of financial populism.

Trust Obama, she was saying, and you can trust me. The crowd at the Wynn hotel and casino in Las Vegas, at least, bought it.

She got a big assist from Bernie Sanders, of course, when he drew huge applause by urging everyone to forget about the "damn emails" and focus on income inequality. Clinton's favorability ratings have fallen since she left the State Department in 2013, partly because an insurgent has emerged to remind anti-establishment Democrats that they have a choice, and partly because she has seemed unsteady in response to questions about her private email server. She needed a strong performance on Tuesday to remind Democrats what they once liked about her, and she delivered.

Going forward, Clinton's donor base will be mollified. Her supporters will be ecstatic. And the rest of the party will see her as a very plausible nominee.

Where, in this mix, can Joe Biden fit? What interests would his candidacy serve, aside from his own?

Biden's candidacy only works on the theory that Clinton is hemorrhaging support, or that her status as frontrunner is shaky, or that she seems unable to articulate a message capable of defeating an emboldened, excited Republican Party in the general election. On Tuesday, her preparation and experience, set against a rather underwhelming cohort of debaters, shows that none of these three conditions is operative.

Clinton still has not fully fleshed out an answer to the toughest question facing her campaign: Aside from her being a Clinton and a woman, why should she be president? But she may be getting there. Fathers being able to tell their daughters that "you, too, will be able to be president," is as close as she has to a reason, and it's compelling. Sufficient? No. "I'm a progressive who likes to get things done," which she said earlier, is closer.

Sanders has drawn significant stylistic contrasts with Clinton, but the premise for his campaign is self-referential too. He spoke of the house parties his campaign was holding, and of the millions of dollars he's raised from small donors, and of the need to "raise the consciousness" of the public at large by presenting it with the facts about income inequality. Sanders' pledge to "mobilize our people to take back our country from a handful of billionaires to create a vibrant democracy" is an applause line that builds its own ceiling.

As former Sen. James Webb said to Sanders, "The revolution isn't coming."