For the first ten years allow free immigration between the new countries so everyone has the opportunity to migrate to the country for which they have the most affinity. Split up the nukes and other military assets so each country can deter attack. The countries will work out taxes, trade, and other economic arrangements as they see fit. Those may run the gamut from Marxist collectivism to laissez faire (where SLL would end up).
The divisions you note are pretty well defined in the later books of the futurist Robert Heinlein. As I recall his book Friday provides the best descriptions of what has been labeled the Balkanized America.
It’s prescient that he realized decades ago the political boundaries that seem to exist exist today and that you have noted.
Conservative vs liberal would be good enough split.
I think maybe down the middle, or at least around the Rockies, then choose which side is which.
Since we conservative love our history, I propose we keep the East which has more of the historic American stuff. The libs can have the west which wont have as many statues and historic sites from the Founding and Federalist periods to tear down, plus they can feel more kin to their Mexican illegal soul-mates in the desert.