It IS totally possible. A red state(s) could secede then form a new federal government plus armed forces and clamp down on the cities. Cities are small area - big population. Ugly yes but do-able.
The blue states own the cities but they can't clamp down on the country side. Ask the Viet Cong about that....
Unconvinced here...
The Taliban pretty much controlled rural Afghanistan.
They could have cut off the food supply to any Afghan city easily.