Seen a few other people cover this, but if the Dems did try to pull this stunt with the governors acting as replacement government. Good luck with that, they'll have control of the big cities and that's basically it. Rural parts being red will break away from the blue cities, and how will they stop it?
Use the national guard (what other forces will they have) that will both be sending people to occupy their hometowns, while also dealing with federal forces? And that's not including things like Southern Illinois using actions to join Kentucky or Indiana instead, would those states National Guard be in play? Let alone anyone that decides to defend their hometown.
Sure Big cities have their advantages in this scenario, but just need to cut off logistics. For inland places like St. Louis, blockade Mississippi River, Checkpoint major highways, and they'll fall quick. Even if flying supplies in remain untouched, that comparative slow drip of supplies with that many different people, is...