I do think Biden would probably have the best chance of taking Trump down, if he were to run. Yeah, he's gaffe-prone (compared to the Orange One, though?
) and he's old. Trump and Hillary and Bernie were all old too.
You do need someone who's going to claw some of those midwest blue-collar Democrat voters back to the Democrats, though. And you don't do that with a Warren or whoever.
Harris might have a shot, I guess Booker's not all that much further left than Obama/Biden. He's a reasonable choice.
But yeah, this sense that "we need to go
harder left this time" is pretty likely just going to get a Trump: Round 2 situation. The Bernie fans make up, what, 20-30% of the electorate on their best day, the rest of the country thinks they're nuts. You've gotta go with a reasonable centrist, and someone who that Obama coalition is going to get behind. Hillary-style, only a good person.