I don't think full secession is necessary or desirable. I'd rather see us take large leftist cities and re-define their metro areas as independent self-governing Federal Districts, like D.C. is today. They get to run their internal politics however they wish, but they don't get to vote in Federal elections anymore.

In a sense, it would be a sort of end-run around repealing the 14th Amendment.

Europe has a number of microstates, like Liechtenstein, Monaco, San Marino, Vatican City, even Luxembourg, I suppose. Something like the microstate model would make a lot more sense for the U.S. than trying to create some set of regional republics where right-leaning rurals are still ruled over by a cosmopolitan leftist imperium.