I'm more focused on electability right now. I want a candidate who will be able to beat Trump. I don't care if they aren't the ideal progressive candidate. Anyone who wants Trump to be a single-term president needs to forget about litmus tests. Right now, Biden seems to be the best bet given the limited polling data we have at the moment. Yes, he's centrist to a fault, but a moderate Democrat is still infinitely better than Trump (or any other modern-day Republican, for that matter). Of course, that could change considerably as we get closer to the primaries. Maybe Bernie will end up being our best bet, or maybe one of the lesser-known candidates as they become better known. But we need someone that's going to appeal to the middle, not just to the base.
Despite the all the news this week, I still think the Democrats have the upper hand. The first rounds of polls after the release of AG Barr's summary of the Mueller Report show that Trump's approval ratings are still underwater, and if they don't improve over the next month or so, I think it's safe to say that his poor ratings had little to do with hypothetical links to Russia. He is still a boorish man-child that can't go a single day without making an ass of himself. He is still a very authoritarian-minded person who idolizes dictators and openly expresses a desire to punish his opponents for having the audacity to oppose him. His policies are still rubbish, and most are hated by a majority of Americans. He never revitalized the Rust Belt despite all of his posturing about bringing back manufacturing jobs. In general, it seems almost like he goes out of his way to be a polarizing figure. If his approval ratings continue to hover in the low 40s in the coming months, then the 2020 election will be the Democrats' to lose. And if his ratings don't improve appreciably within the next several months, then I doubt they will improve by this time next year when the primaries are in full swing. No President with net negative approval ratings has been re-elected since approval ratings have been a thing, and if Trump continues to be deeply unpopular into 2020, his chances of winning become ever more remote.
However, the Dems absolutely cannot rest on their laurels regardless of how weak and vulnerable Trump is as a candidate. They need to buckle down and take this election seriously. They need to accept whatever the final Mueller report says (assuming we ever get it), and if there's still isn't anything definitively demonstrating some kind or criminal or ethical malfeasance on Trump's part, then they need to absolutely drop that and focus on the rest of Trump's insufferable bullshit. Trump has done enough to earn the ire of a majority of Americans, and Democrats cannot squander this. All indications, from Trump's state-level approval ratings to hypothetical 2020 match-up polls to the results of the 2018 midterms, suggest that we've rebuilt the "Blue Wall" and that Arizona and Georgia will likely be in play after being reliably red for years, so they need to continue to focus on those areas to secure or flip them into the Blue column. We could even put Trump on the defensive in Texas.
Whoever the nominee is, they need to play it smart, and the base needs to turn out to vote this time and not sit at home or vote third-party if Bernie doesn't win in the primaries again. Defeating Trump should absolutely be our top priority next year, not ensuring our ideal progressive candidate becomes the nominee.
Biden has one fatal flaw, though: He tends to screw up on the way to the finish line, that's how he lost his primaries before. Obama choose him as VP to show that he has a thick skin after the comments he made over him (his comment was: "I mean, you got the first mainstream African-American who is articulate and bright and clean and a nice-looking guy, ... I mean, that's a storybook, man."), and that could bite him hard if that gets digged out by right-wingers. And Trump is somebody who would exploit the hell out of his flaw, so not sure if he really would win against Trump in the end.
Bernie Sanders seems like a more probable winning candidate on the long run to me right now. Both him and Biden do win every national poll against Trump to date (which Warren, Harris or Booker can't claim), so he's certainly not just appealing the base. And he's known for staying steadfast to his convictions, so he won't flip-flop on issues, either, so that will be hard for Trump to play against him.