Maybe the solution is for the largest cities to exist as semi-independent city-states. The citizens there cannot vote in state or federal elections, but they can have their own constitution and vote themselves all the dictators they like.
Such cities would be one-party states run by rich oligarchs, basically like now. They could do almost any progressive thing they wanted, short of executing political prisoners. Instead, they could exile their dissidents (we would thus get a stream of their best people). Meanwhile the worst people on our side would leave to get the free recreational drugs or whatever the Socialists promise. We could trade with the city-states and even have a common currency.
The largest “blue” cities and their immediate surrounding blue or deep purple areas could be these city-states. There are some cities that are a mixture and there are smaller blue cities where this would not work.
I don’t think anyone should have dual citizenship in my proposed city-state and in the United States.