I'm interested to see this, as I think the idea of an American Civil War gets thrown around, but as many have described here, rural vs. urban seems to be the closest thing to a geographical division.
I think an interesting version of this is in Omar el Akkad's American War. I'm not remembering all the details, but it was a small cluster in Alabama, Mississippi and Georgia that split, but while it's obviously not going well for them, they muck it up and use terrorist tactics while climate change and plague are causing chaos, and there is major foreign support for the breakaway region to destabilize the US.