I had the pleasure (?) of knowing several 'Bernie Bros' in 2016, and they seemed more interested in getting 'revenge' (for what I don't know) against Clinton than in defeating Drumpf. There was even one coworker who pointedly said that as a white man, he 'didn't have to worry about anything anyway.'
As for Sanders, if he wants Harris to win, wouldn't it be more productive for HIM to speak forcefully about how much better she will be for the working class, and everything that the Biden administration has been working on for almost 4 years?