The question is what accounts for the decline. Why is Sanders falling so much shorter this time around?
It may well be that the most significant factor in the answer is Hillary Clinton. Clinton was by no means universally loathed by Democratic voters, but she may have been strongly enough disliked by certain (working-class white) voters that they cast ballots for Sanders in the primaries as a kind of protest against the party’s presumptive nominee. That could easily have been enough to raise Sanders’ vote tally by 10 or so percentage points in comparison to this year, when his main competition (Biden) is much less widely disliked among the same demographic group that was repelled by Clinton.
Then there’s the electability argument and the likelihood that significant numbers of primary voters this time around simply can’t imagine that an outspoken socialist proposing tens of trillions of dollars of new spending will be capable of beating President Trump in November.
Finally, there’s the gathering storm of the global pandemic that has turned American life upside down and inside out over the past two weeks. Any lingering chance of Sanders turning things around after Super Tuesday were almost certainly lost as epidemiological and economic fears have spread throughout the country, arousing a longing for the return of basic competence and a steady hand at the helm — a longing that a mainstream, veteran pol like Biden is perfectly situated to fulfill.