The dems think that Trump is so beatable that they wouldn't have to bring in anyone super-dynamic to beat him. They may have come to the conclusion that any democrat could beat Trump, except for one as laden with baggage as Hildabeast. (I'm only saying what I believe THEY think.) So, someone deemed harmless, like Uncle Joe Biteme might be brought off the bench. And one hears Faux-co-hantas' name bandied about. Of course, if Bernie isn't the heir apparent to the disgraced front-runner, the Bernie-bots will go absolutely berserk (which, for many of them, ain't that far to go.)
Living here in the People's Republic of New Jersey, I well remember a somewhat similar situation in the 2002 Senate race. Robert Torricelli, the democrat candidate, was so mired in scandals related to bribery, graft, corruption, pay-to-play and the like that he was actually trailing his republican candidate by double digits, not a normal situation in this very blue state. Despite the fact that the deadline to switch out a candidate had long since passed, the dems pulled Torricelli and replaced him with the late, avuncular, Frank Lautenberg. (He wasn't actually "late" at the time, although opinions differed.) Link.
The switcheroo was brought to the NJ Supreme Court, a deliberating body that is only slightly to the left of one you might encounter in Cuba, Russia, or among kangaroos in Australia. Of course they agreed that the switch was legal, that the words in the law didn't mean what they said (John Roberts, anyone?) and allowed old Frank to be the candidate. The democrats immediately went from down 12 to up 12 in the polls, and Lautenberg won going away.
“...the Bernie-bots will go absolutely berserk (which, for many of them, ain’t that far to go.)”
About as far as from the den to the bathroom. If it were any farther they wouldn’t be Bernie-bots.