The polls have only recently closed on the West Coast as I write this up. At this point, we know that former Vice President Biden (the one-time frontrunner turned underdog) has run the table in the South, winning Virginia, North Carolina, Tennessee, Alabama and Arkansas. These states, all of which have a significant African-American population, were always supposed to be good for Biden. But, they were also states that Mike Bloomberg, thanks to his millions of dollars of advertising, was threatening to win, or at least cut-down Biden's margins. That didn't happen. Biden also carried Oklahoma, a state that Sanders won decisively in 2016. 

More important, Biden won Minnesota, a much less diverse state where, not that long ago, polling showed him not even cracking the 15 percent threshold. He is also holding his own in New England — the backyard of both Bernie Sanders and Elizabeth Warren. He won Massachusetts and may win Maine as well.

Texas was also called for Biden. This was a state where Sanders was counting on building up a big delegate lead. California, the big prize of the night, is likely to go to Sanders, but not by the margin it once looked like it could.

So, what happened?

1. Electability has always been the driving factor in Democratic nomination.

For months, Democratic voters told us in person, in polls and in focus groups that they didn't care who won the nomination, they just wanted it to be someone who could beat Donald Trump. The fact that ideology or policy differences were taking a back seat to a proven ability to defeat Trump was supposed to simplify things. Instead, it paralyzed voters and the Democratic candidates. They were all so worried about messing up, that they were unwilling to overlook flaws and weaknesses. They were desperate to find a perfect candidate who, we all know, doesn't exist.

For months, Biden led the polls on the question of who is the most likely to beat Trump. But, his shaky debate performances, listless campaign operation, and, ultimately, losses in places like Iowa and New Hampshire took him from the "safest choice" to a risky bet. 

But, as Sanders started to pile up wins — and delegates, the Vermont senator became a riskier proposition. Sure, Biden had lots of flaws, but, many Democrats thought Sanders' nomination would cost Democrats more than the White House; it would mean losing the House and forgoing any chance at the Senate. 

Once Biden came up big in South Carolina, Democrats Amy Klobuchar, Tom Steyer and Pete Buttigieg did something that the GOP establishment wasn't able/willing to do in 2016: unite around the one candidate who could realistically stop a Sanders nomination. One GOPer remarked to me earlier today that it's a sign of just how much better organized and bold they are than the GOP. But, I would argue that it's really much more about this moment in time than anything else. To Democrats, Trump represents an existential threat that overshadows any and all other things that would keep them from such a remarkable push of unity in such a short period of time.

And, that unity push paid off. 

Exit polls show late deciders breaking overwhelmingly for Biden; Biden won them by 39 points in Virginia, 41 points in North Carolina and by 34 points in Minnesota. Why was Maine, so close? Sanders won voters who had decided their vote before this week by 32 points. But, Biden won late deciders (who were 45 percent of the electorate) by 22 points.

2. Donald Trump is the Democrats' best GOTV. 

The other thing that Biden lacked that literally, every other candidate had more of than he did: money and organization. Sanders and Warren had invested in field operations and the early vote for months. Bloomberg, of course, spent more than $500 million on TV ads and campaign infrastructure. But, in the end, Biden had the more powerful GOTV operation: Donald J. Trump. Democrats want to beat Trump. Biden is the guy who is most likely to do it. The end. 

3. Biden owes Warren a huge debt of gratitude.

While Biden's win in South Carolina was obviously a huge factor in his comeback story, Mike Bloomberg's debate flop before the Nevada caucuses was a bigger factor. Before that debate, Bloomberg was seen as the candidate who could go the distance against Sanders. He'd have the money that no other candidate could imagine. And, he was putting together a coalition that included voters of color, especially African American voters. But, his weak performance on the debate stage — especially his inability to effectively parry the attacks coming at him from Warren, undermined any credibility Bloomberg had to label himself the "Trump killer." It just wasn't believable. All of a sudden, Bloomberg looked like the riskier bet than Biden.

4. The kids are alright. But, they aren't turning out in record numbers.

The Sanders argument for electability is simple: only he can turn out the younger voters and especially the younger voters of color who stayed home in 2016. But, if that's the case, we haven't seen much evidence of this surge of young voters into the primaries. 

In the 2016 primary, for example, 42 percent of the Texas electorate was under the age of 44, and 58 percent was 45 years of age or older. According to the exit polls tonight, just 38 percent of the electorate was under 45. We may see those numbers change as we get more data in, but at the very least, Sanders hasn't expanded the universe of younger voters. 

For much of the year, we've also been asking whether Sanders has a ceiling. In Iowa and New Hampshire, where Sanders took just 26 percent of the vote, it looked like the answer was yes. But, his 47 percent win in Nevada suggested that we had underestimated him. Tonight, however, that ceiling was once again, very evident. With lots of votes still out, it looks as if Sanders' largest margin will come in Colorado, where he is taking 36 percent of the vote. Otherwise, he'll end up, in most of the Super Tuesday states, with vote share in the mid-20s to low-30's. 

In the next hours and days, we'll have a better sense of the final delegate count, but at this point, the once unthinkable has happened: Biden will end Super Tuesday with more delegates than Sanders. 

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