That electability is extremely difficult to measure doesn’t mean it’s impossible to measure altogether. This study is not encouraging:
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.
The large number of young people who say they will only vote if Sanders is nominated is just enough to offset the voters Sanders loses to Trump in the rest of the electorate. (Warren appears to lose at least as many Republicans as Sanders, but does not seem to benefit from any compensating enthusiasm from young voters.)
Sanders himself has been clear that his strategy for beating Trump is to massively boost turnout, especially among young people — and young people in our data indeed say they would turn out at much higher rates for him.
But for Sanders to do as well as a moderate Democrat against Trump in November by stimulating youth turnout, his nomination would need to boost turnout of young left-leaning voters enormously — according to our data, one in six left-leaning young people who otherwise wouldn’t vote would need to turn out because Sanders was nominated. There are good reasons to doubt that Sanders’s nomination would produce a youth turnout surge this large. . .
If the turnout of all age groups increases from 2016 to 2020 (as happened from 2014 to 2018), then the turnout among young people must increase by 11 percentage points above and beyond this broader trend, and must do so solely due to Sanders’s presence on the ticket. Finally, youth voter turnout doesn’t usually go up or down by nearly as much as 11 percentage points from election to election; the Sanders boost would have to be truly unprecedented.
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.
This is completely outside my areas of expertise, so I hope someone has a good argument why Broockman and Kalla are wrong, or at least overstating their case.