The primary political division in the US is urban versus rural, with progressives concentrated in the cities.

There won't be a confederacy. "MAGA America" would need to cut power and communications to the major cities. Bonus points if they can also disrupt fuel, water, and food. A few hundred people could accomplish that in one day, without ever having to face the military or occupy a square inch of land. Cut the urbanites off for more than a week, and they have effectively won.

Urban America needs to guard millions of miles of power, communication, fuel, water, and rail lines. Thousands of miles of highways. Every bridge, tunnel, and transportation chokepoint leading into the cities. And occupy enough farm and ranch land to keep themselves fed. Without getting killed.

Urban America may be a majority, but don't be deceived into believing they have that much manpower. They don't.