Obama has a chance to be statesmanlike and pick someone widely respected--the closest thing to a Scalia on the Democrat side that he can find. But that would require him to think about what is best for the country and for his legacy. He is very unlikely to take that route when the alternative is to make a political battle out of it in hopes of scoring some advantage short term from it. Anyway he will have to do whatever Valerie Jarrett tells him to do.
I don’t care if their Rat opponent is to the left of Bernie Sanders, any Republican Senator who doesn’t stop this will get voted out.
With all the Republicans loudly announcing they will oppose anybody he nominated they handed Obama a clear win. He can do just what you suggest, nominate a well-respected left-center candidate, then use opposition to that nominee in the Senate races to try to get a Dem majority in November.
The Republicans outmaneuvered themselves without the opposition moving.