I think they are staking it all on vote harvesting and mail-in fraud. So, the popularity of their candidate means nothing. No one likes Hillary? Who cares? Michelle doesn't want to do the work? [shrug] Elect Michelle and Valerie Jarrett will do all the work.
I am being flip. But my overall point is serious: the Democrats do not need an electable candidate this year. They plan on stealing it. They do not have a Plan B. They just need someone in the chair who won't cause a problem. Joe is someone who would cause a problem.
Biden is exactly the rubber doorstop the likes of Blumenthal/Podesta/Soros/Brennan/etc. want as president. If they can figure out how to avoid debates with minimal damage (the press will oblige) they can vaccuum the oval office rugs around him while he sleeps and they launch ME wars.