They think that by keeping him on the ballot the votes will go to him instead of Trump
No. They think that by keeping him on the ballot, they can justify votes redirected to the Dem candidate.
The registered voters and total votes are what they have “to work with”. They will do whatever looks plausible.
Three years ago I drove Michigan Detroit to the UP there were hundreds of huge Trump signs along the highway. I bet they are still up and voters ready for Nov.
Democrats can’t do anything right. This will backfire on them.But the ballots are probably already filled out and printed.