Having Biden as the nominee will allow them to use the Ukraine phone call/impeachment farce as election propaganda...they will figure that people won't remember the details of it all by November so they can just bombard people with the idea that Trump should have been removed from office and only survived because the Republican senators protected him.
Dementia is a funny thing. You can go on for awhile...and then just go down hill really fast
Dont count on them being able to hide it come November.