Even though there were 20 candidates in the mix over the two nights of the first Democratic debates, Thursday's second-round debate in Miami was bound to be focused on former Vice President Joe Biden. For months, he has being treated like the presumptive nominee in the media, thanks to his clear lead in the polls, at about twice the support of Sen. Bernie Sanders, who has consistently placed second.

But, as University of Wisconsin political science professor Kenneth Mayer warned viewers in a Salon interview, primaries tend to be more volatile than general elections. He noted "that in December 2003, the month before the Iowa caucuses, the top three Democratic candidates were Howard Dean, Dick Gephardt and Wesley Clark — and within a month they were all out."

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Right now, Democratic voters — still traumatized by Hillary Clinton's 2016 loss — are laser-focused on the ambiguous quality of "electability," which Biden is perceived to possess, largely due to his proximity to the last Democrat elected president, Barack Obama. Obama played many roles in Biden's pitch on Thursday. Sometimes, he was Obama the Redeemer, washing away Biden's past sins, such as his vote for the Iraq war. Sometimes he was Obama the Father, giving guidance to Biden, who loyally carried out the orders.

Sometimes Biden even seemed to imply he was Obama, taking credit for many of the administration's successes, even when, at least in some cases, he was the voice on the inside arguing against those policy choices.

For the first half of the debate, this strategy seemed to work well enough. Most of the other candidates, jostling for a chance to make positive statements about themselves, didn't waste their breath attacking Biden. And so Biden was able to shrug off digs about his age from Rep. Eric Swalwell and criticism about Obama's immigration policies from Sen. Kamala Harris, maintaining his cool and sticking with Obama's-third-term pitch.

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Until, that is, Harris finally drew blood in the second half, tearing into Biden for his recent comments about working with segregationists, and effectively tying that rhetoric to Biden's real-life efforts to stop federally-mandated desegregation efforts in the 1970s. In a move that particularly tore at Biden's collegiality-is-everything philosophy, Harris made it personal, noting she was a beneficiary of the busing system as a child in Berkeley, California, bringing emotion to an issue he has mostly treated as an abstraction.

Harris playing Tom Cruise in "A Few Good Men" worked and Biden stepped directly into the Jack Nicholson role, launching into a defensive rant about how he was misunderstood and that his anti-busing stance was just about — well, he didn't say "states' rights," but he toed that line.

After that, Biden never really recovered, and in fact, got worked into a defensive rant all over again when Sen. Michael Bennet criticized him (fairly) for his role in extending the Bush tax cuts.

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The night was rough for Biden, but if there was a clear winner, it was Harris, and not just for landing the cleanest punch on the frontrunner. From the beginning to the end of the debate, Harris managed to communicate clearly that she was the total package: Smart on policy, tough in politics, but driven by a clear sense of empathy.

More than any other candidate, Harris spoke in the emotional language that, for better or worse, drives politics. She always brought in evocative language about the real people affected by the issues being discussed on stage.

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But she also conveyed a toughness, from the first question when she called out media double standards on the "How do you pay for it?" question that is always asked about Democratic policies but never about Republican ones.

Early on she quipped, during a moment of aggressive crosstalk between numerous candidates, "America does not want to witness a food fight, they want to witness how we're gonna put food on their table."

Later, when she interrupted a discussion of racism and police violence, she noted that as the only black person on the stage, she felt entitled to speak "on the issue of race."

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And in her closing statement, drawing on her background as a prosecutor, Harris described herself as the candidate who "can prosecute the case against Donald Trump."

"Electability" is a hard-to-define quality, so it's not a surprise that voters who know very little about the candidates right now, are relying on familiar stereotypes (such as: white + male = electable) or simply on name recognition.

But Harris has now made a compelling argument for herself: She's both a counterpoint to Trump, in that she has compassion and clarity of vision, but she's also the candidate with the gonads to stand up to the old man. As she did to Joe Biden Thursday night.

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The other candidates on stage probably won't be remembered much, except for oddly-accented spiritual healer Marianne Williamson and her flights of fancy. Sen. Kirsten Gillibrand, who is generally better than this, fell flat on her face, interrupting too often and being excitable and nervous when she did manage to talk. Her campaign is likely almost over

As for Sanders, he did a fine job, forcefully hitting his marks and repeating his long-standing talking points about wealth and economic justice. But what used to feel like more daring rhetoric now blends into the wallpaper: Other candidates have embraced much of his analysis and policies, and are blending it into a more compelling presentation. His closing statement, in which he implied he was the only real lefty in the race, fell flat, since he failed to acknowledge how much the party has changed since he first started spinning that tune. But if Sanders gained no ground, he probably didn't lose any either, which is more than might be said for Biden.

The one candidate who had a hard-to-categorize night was South Bend, Indiana, Mayor Pete Buttigieg, who has been in a buzz-backlash cycle among elite political junkies while being ignored by most everyone else for a couple months now. For most of the night, he did just fine, presenting himself as a standard Democrat and holding his own with more seasoned politicians without standing out.

But he did have one important moment that stood out from the boilerplate, when asked about the killing of Eric Logan, a black South Bend resident recently shot by a white police officer. Instead of taking on Biden's "you don't know my struggles" method, Buttigieg struck an apologetic tone, taking responsibility for failing to do more and saying that "all of the steps that we took from bias training to de-escalation" weren't enough to "save the life of Eric Logan."

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That was undercut when Swalwell sniped at him, "You should fire the chief," and Buttigieg responded with a bureaucratic excuse that may well be true, but didn't sound good under the circumstances. Still, simply by not being messy and defensive, Buttigieg came off far better than Biden, though it may not do much to boost him in the polls.

Unlike the first debate night, which was largely polite, there were some real fireworks in Thursday's debate. But the important thing is the definition of "electability" might be shifting. Biden's vulnerabilities got some national exposure and, perhaps more important, Harris worked hard to recast what qualities might make a candidate "electable." If she keeps that up, she might just win this thing.