The main argument given by the mainstream Democrats given for supporting Biden vs Sanders is "electability." Democratic party leaders are openly willing to risk party damage to stop Sanders all because of "fear that [Sanders] will lose to President Trump and drag down moderate House and Senate candidates in swing states." 
Cherry-picked polls, usually party-sponsored, are used to support this argument, and party line pundits endlessly promulgate it. But there is better evidence than party-sponsored polls and pundits. Gamblers do not blow hot air to spin a subject; they don't care to persuade you of what they believe. (More likely, they would try to persuade you otherwise, to get you to make a sucker's bet). Rather, they put their money on the line. And as one might expect, money-lines predict election results better than polls or pundits, just as they do in sports and other areas in which gambling markets exist.
And what do the gambling markets say about "electability"?
Well, first, they say that it, in fact, mostly a two-man race, and that based on recent events, Biden is now more likely Sanders to win the Democratic nomination. Betting odds as of 8 a.m. indicate that: 
* Biden has a 47.8% probability of getting the nomination (+100 Biden; -120 field)
* Sanders has a 44.7% probability of getting the nomination (+115 Sanders; -135 field)
Yet despite a significantly lower chance of getting the nomination, Sanders has a significantly higher chance of actually being elected President:
* Sanders has a 21.7% probability of being elected President (+345 Sanders; -430 field)
* Biden has a 21.0% probability of being elected President (+360 Biden; -450 field)
Putting the two together, wagers indicate contingent odds that:
* if Sanders gets the nomination, he has a 48.5% probability of actually winning the presidency (21.7% / 44.7%)
* if Biden gets the nomination, he has only a 43.9% of actually winning the presidency (21.0% / 47.8%)
So to the degree these things are in fact predictable, not only is it wrong to say that Sanders will hurt Democratic chances in November, the most reliable predictors indicate that a Sanders nomination will significantly help Democratic chances.
 NY Times "Democratic Leaders Willing to Risk Party Damage to Stop Bernie Sanders"; updated/accessed 3/2/2020 2:53 p.m. ET, click here
 Odds are from .5dimes.eu/ as of 8:00 am Tuesday 3/3/2020.
(Article changed on March 3, 2020 at 15:59)
(Article changed on March 3, 2020 at 20:18)
(Article changed on March 3, 2020 at 20:28)