As for the current candidates, Bernie Sanders and Elizabeth Warren's socialized economic policies come off as so far left and out of touch that I don't think either candidate is electable. The tuition-forgiveness and free healthcare for everyone, including immigrants, are just going to be deal-breakers.
Joe Biden is the senior statesman among the Democrats and a relatively calm voice of reason as a centrist, but the left wing of the party does nothing but attack him. Plus, he's showing signs of age to the point he occasionally comes off infirm. It seems the Dems are looking for someone who is younger and sharper to emerge.
The youthful and openly gay candidate Buttigieg doesn't come off as particularly strong, articulate or definitive in his views on policy. He comes off as a novelty candidate representing social progressivism, but I don't think he'll hold up in an election.
I like Tulsi Gabbard generally, and think she's personable, relatable and has genuine conviction, but she's back in the pack of candidates.
At this point, I think the Dem party is going to try to push Harris to the front. I could see her carry several blue states, including Calif., Washington, Oregon, Massachusetts, and New York, but I don't think she'd get much farther than that.
I think the Dems realize they're not in a strong position for the 2020 election, and they're going to keep attacking Trump with Mueller and anything else they can muster. In the end, I think Trump gets re-elected with roughly the same electoral majority he had last time, or a bit more.