South and parts of West for Republicans, Northeast and West Coast for Democrats. Midwest a bit tricky, but doable. Make it amicable, establish trade deals, border controls and see what happens.
A fly in the oil could be global warming and drought. Louisiana might be a tough place to live in 50 years from sea rise and tropical diseases. Arizona could be uninhabitable from heat and lack of water. But those folks won’t have Biden to kick around anymore.
One twist is that as climate change heats up, droughts turn to desertification, the new nation may send refugees to the North. Will they be accepted at the border?