I once imagined a scenario, following some sort of unprecedented armed conflict of short duration (such as @Mops envisions), whereby the warring factions would realize that their country had been split more or less 50-50 for generations, and that the only peaceable solution was for them to form two countries. I drew up a map of where the borderlines would be, making both “countries” equally unhappy. Basically, this map shows the East coast from coastal Maine down through roughly Wilmington NC aligning itself with Canada, and also the northern part of the country also joining Canada, roughly from Buffalo through Minnesota. The entire West Coast would also join with Canada with an arm of that coast sweeping through to some big cities, perhaps Phoenix through parts of New Mexico and bending as far north as Boulder CO.
The rest of the country would be a contiguous odd-shaped agglutination of southern, mid-western, and far western states, still with some big cities intact (sorry, Omaha, Houston, and St. Louis—also, sorry Denver and Albuquerque if you get left behind too.) The guiding principle for being able to join one country or the other is connection to the main body, the principle being “No isolated cities, towns, counties, villages—everything must have a physical path to the main body of the country it belongs to (except for actual islands off the coasts).” The two states that are outside the contiguous U.S. to begin with, Alaska and Hawaii, can make their own decisions—pretty sure we can predict how those choices will go.
There will necessarily be large-scale repatriations of left-wing Democrats trapped in rural Iowa, of course, and right-wingers stuck on the west side of Manhattan, but no dual citizens. You would declare your loyalty to one country or the other. No pledge of loyalty, no citizenship at all.
Repatriation is key. Those of you complaining that you’re being forced to choose between your core values and the part of the country you love, your family, your job, your culture, the weather you like, et cetera, you’re absolutely right. You have to suck it up. Are you better off living where you live or living in a society that supports you on immigration, abortion, separation of church and state, and a few dozen other major issues? You’ll be supported by the government seeking to repatriate you but if you refuse to move, you’ll spend the rest of your life as a marginalized quasi-citizen who never comes close to winning an election (or perhaps to having an election) again in your lifetime.
It will take an awful lot to get us there, but I think that’s where we’re heading, in another 50 or 100 years of this shit.