The Bernie Sanders campaign was built on big promises. There was the promise of affordable health care, the promise of free college, the promise of a higher minimum wage.

For voters who didn’t need, or want, those promises, he offered something else: That he alone could expand the electorate and win back voters who had turned away from the Democratic Party and embraced Donald J. Trump in 2016.

The coronavirus crisis turned everything that Mr. Sanders promised he was best equipped to do — fix the health care system, call out the dangers of a Trump presidency — into an agenda that was more urgent than ever for the country.

It was the moment for his message, but he was not the right messenger for the moment.

His promise was inherently risky, one that required, in his words, nothing less than a political revolution. And that is where the Sanders campaign fell short: In a world that had seemed risky enough already, revolution was just too much.