If Hillary wins and the Democrats take back the Senate then Obama's nominee is probably no worse than what will get crammed down in January. Otherwise, vote the nominee down on a party line vote and let President Trump or Cruz pick a new nominee.
There are some advantages to your suggestion. But let’s say obama’s preferred leftist is a black. Dems can then use that pending appointment to motivate what right now looks to be a very uninterested black vote this November. Without the black vote, dems don’t win.
Better to eliminate the political angle right now by clearly stating that, because it is an election year and the american people should decide the appointment, we will not approve anyone obama nominates no matter who it is.