If the red states secede, it will start with Texas.
The Republican President meets with the Republican members of both houses and the conservative Justices.
They vote to expulse the following cities from the Union:
Boston, New York City, Philadelphia, Chicago, Detroit, Miami, Las Vegas, Seattle, Portland, San Francisco, Los Angeles. (Not an inclusive list and by county). New Governors will be appointed to those states subject to a special election. The remaining state legislatures will elect new senators. Original blue Senators and Representatives would lose their credentials. The President would appoint replacement SC jurists and the result would be as follows.
Most of the country would be intact with isolated heavily populated counties mostly on the coast broken off and maybe some New England states. It would contain slightly more than half the US population. It would contain most of the military and almost all of the states. The Blue areas would suddenly find themselves out in the cold they can try to unite to fight the Red menace but they would be in large isolated from each other. Maybe Central Southern Cali might form a country and New England NYC and NJ another. Their populations would decline rapidly as the illegals bail out and other legitimate immigrants decide they don’t want to move into a civil war. The Red USA would control prices on Electricity, Gas, Food, Water and can elevate or reduce the prices depending on the aggressive nature of those cities.
Also border gates and walls would go up delineating the separation. Within ten years the Red USA population would out number the Blue Cities populations and some cities would file and present documentation to petition to reenter the US subject to Congress’s approval and the President’s signature. The US of course still owns harbors airfields army bases and Federal buildings in Blue areas.