It is actually fairly neat geographically. Draw a line 50 or so miles inland on both the West and East Coasts. The two coasts are one country, the rest is another. A few million would be trapped behind enemy lines on both sides, but they can either accept permanent minority status or move.
No, actually, it’s not. Practically all the coastal counties of North Carolina are red, for instance. Get away from the oceanfront counties and they’re practically all red, military bases. There are red coastal counties in, get this... California.