Could be the donors, could be the party, could be a true believer.
I don’t listen to his show so idk the reasoning but this is probably one of those stopped clock situations.
They lose with Biden, if it goes instead to a brokered convention they lose. If Biden dies then there’s a good chance they win.
Your question of who is the sticking point though. The party doesn’t care about winning, the donors are all giving to both sides anyway, only a true believer would do such a thing.
There’s not a lot of precedent for that though. There was the congressional baseball watcher, but even then it was an ill conceived haphazard attack with little preparation.
Pardon the pun, but democrats just don’t got any shooters.