Well, as Ronald Reagan said in another context and another debate long ago: "There you go again". In the debate tonight, Sanders once again gave his electability pitch and it was exactly the same as it has been all campaign: a gigantic, humongous surge of turnout that will sweep all the Democrats' troubles away. I've been whacking away at this ridiculous assertion as best I can and, coincidentally released the same day as the debate, we have a new and thorough political science study (summarized on Vox) that makes clear just how heroic and unrealistic Sanders' assertions are.
The study, by David Broockman and Joshua Kalla of Berkeley and Yale respectively, essentially shows that Sanders' seeming electability in trial heat surveys--where he runs as good or better than "moderate" candidates--is attributable to two dynamics: (1) Sanders losing voters to Trump relative to moderate candidates but (2) making this up as people who say they are neither/third party/wouldn't vote with a moderate candidate move to Sanders. The net of these two trends keeps Sanders afloat and "electable".
Could this happen in real world? Probably not. The problem is that the implied turnout increase for young voters--who are the ones who come off the sidelines in a Trump-Sanders trial heat-- is wildly implausible. As the study notes, Sanders would have to generate a larger increase in turnout among young voters than Obama managed to generate among black voters in his historic 2008 election. Looked at another way, youth turnout would have to not just go up 11 points but 11 points more than everyone else goes up in the 2020 election. So if turnout goes up 11 points in the rest of the population in 2020, youth turnout would have to go up 22 points. you get the idea.
So, more evidence that Sanders' theory of the case on how he would beat Trump is furshlugginer (look it up).
Some excerpts from the researchers' Vox summary of the study:
We found that nominating Sanders would drive many Americans who would otherwise vote for a moderate Democrat to vote for Trump, especially otherwise Trump-skeptical Republicans.
Republicans are more likely to say they would vote for Trump if Sanders is nominated: Approximately 2 percent of Republicans choose Trump over Sanders but desert Trump when we pit him against a more moderate Democrat like Buttigieg, Biden, or Bloomberg.
Democrats and independents are also slightly more likely to say they would vote for Trump if Sanders is nominated. Swing voters may be rare — but their choices between candidates often determine elections, and many appear to favor Trump over Sanders but not over other Democrats.
Despite losing these voters to Trump, Sanders appears in our survey data to be similarly electable to the moderates, at least at first blush. Why? Mainly because 11 percent of left-leaning young people say they are undecided, would support a third-party candidate, or, most often, just would not vote if a moderate were nominated — but say they would turn out and vote for Sanders if he were nominated....
[T]he “Bernie or bust” phenomenon appears almost entirely limited to left-leaning young people, who are usually a small share of the overall electorate. This stands in contrast to many theories of Sanders’s electoral appeal: For example, whites without a college degree — a demographic some speculate Sanders could win over — are actually more likely to say they will vote for Trump against Sanders than against the other Democrats. The same is true of the rest of the electorate, except left-leaning young people....
The case that Bernie Sanders is just as electable as the more moderate candidates thus appears to rest on a leap of faith: that youth voter turnout would surge in the general election by double digits if and only if Bernie Sanders is nominated, compensating for the voters his nomination pushes to Trump among the rest of the electorate.
There are reasons to doubt a Sanders-driven youth turnout surge of this size would materialize. First, people who promise in surveys they will vote often don’t, meaning the turnout estimates that Sanders’s electability case rests upon are probably extremely inaccurate. Second, such a turnout surge is large in comparison to other effects on turnout. For example, Sanders would need to stimulate a youth turnout boost much larger than the turnout boost Barack Obama’s presence on the ballot stimulated among black voters in 2008....
And this enormous 11 percentage point turnout boost is only enough to make Sanders as electable as the more moderate candidates, given the other votes he loses to Trump. For him to be the most electable Democratic candidate based on his ability to inspire youth turnout, Sanders’s nomination would need to increase youth turnout by even more....
There is no way to be sure whether Sanders’s nomination would produce this historic youth turnout surge — but it seems doubtful. Turnout in the 2020 primaries so far has not exceeded 2008 levels, including among young voters. If anything, research suggests the opposite is more likely to occur: In response to an extreme Democratic nominee, Republicans could be inspired to turn out at higher rates to oppose him.