Full disclosure: In my opinion nothing is more important than ending the Trump presidency. Only a Democrat would have a chance at beating Trump (sorry third party candidates). I would have supported any Democratic nominee. I am less nervous about Biden's chances than I would have been with some of the other contenders.
Coronavirus curtails college campus campaigning
Bernie Sanders' strength has been with college students. With the Coronavirus pandemic, the Sanders campaign couldn't rely on its usual, successful formula. College campuses are like ghost towns with the students taking their classes online from remote locations. There isn't the same chance this year to register students, hold on campus rallies and mobilize get-out-the-vote drives.
A general election strategy of attracting the left and the more left
The rationale for Sanders as the Democratic standard-bearer is that he could best appeal to progressives. The progressive wing of the party is dominated by college students. Sanders pitch was aimed in large part directly at college students, including a promise of free college and the Green New Deal. Harnessing the energy and enthusiasm of college students with the campuses shuttered would be tall order for Sanders. Young voters are less likely consider the Socialist label as a disqualifier. Maybe Sanders would have been successful against Trump regardless of the obstacles.
A traditional general election strategy of attracting voters in the middle
The theory for Joe Biden as the Democrats' nominee is that as a centrist, he would be successful with more middle-of-the-road voters including independents, moderate Republicans, seniors, rural voters, and Black voters who tend to be conservative Democrats. Biden is doing well with suburbanites Republican leaners and disgusted "classic" Republicans like the Lincoln Project. It's questionable that Sanders would have the same cross-over attraction. Biden also has the strategic advantage of being native Pennsylvanian and a Pennsylvania neighbor. He's doing well in the Key Stone State which seems to be the key to winning the election this year. Biden is not a lock, but it's a lot better to up in the polls and flush with cash than to be in Trump's position.
What we will never know and what we will know soon
We will never know how Sanders might have done against Trump. It will be a couple of weeks and few days before we know how Biden will do. If Biden wins and the Democrats can gain control of the Senate, Sanders will be a good position to influence legislation. That is the best of all possible scenarios.
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