Yes, they have sorted and concentrated themselves into the coastal enclaves, Chicago and the far NE. Trump may win with a popular vote minority again quite easily in 2020.
Yup. The urban cores are old and every one who can move out of them is doing it.
The shift to the suburbs and small towns is only going to erode the Democratic base.
If they don’t find a way to expand it, they will remain a minority party.