The democrats WILL nominate a completely bat$#!+ crazy socialist demagogue for President in 2020. She will make Hildebeeste look sane by comparison. The contest between her and Trump will be epic.
This, I think, will be by design. The Dems either know they can't beat Trump in 2020, or that they only way they can get rid of him is either impeachment/resignation or assassination. Nominating a 2020 version of Mondale feeds the alt-left base, and gives the Dems time to craft a Clinton- or Obama-type candidate for 2024 to retake the WH.
This is why I believe Michael O will step forward and run...to save the party from itself. Idiotic white women will put her over the top for the nomination. If the current crop continues forward to the nomination, the election is his to lose. Then hopefully he can put another two or three judges on the court, replacing RBG and SM (who is fairly ill with diabetes). We could also get some of the older conservatives to retire safely and perhaps impeach and replace Roberts. The SCOTUS is everything.