@ossobuco,
I see the unions heading back to Biden, who's been their guy for ages. If he manages to pull enough primary votes to dump Clinton, then the Democrats are going to have rethink an entire plan. Hopefully they've got it in them.
There is a subset of Sanders supporters who are not Democrats - will they swing back to the Republican party or simply not vote if Sanders is out? not voting is obviously the better of those two options (from the perspective of a supporter of the Democratic party).
There is a series of anecdotal reports of traditional Republican voters who have decided they're independent now. Who will they vote for? Can the Democrats figure out a way to get those new independents to vote centrist vs right?
With another possible senior candidate, what are Democrats doing to develop younger candidates who will be appealing 4 years down the line?