You have to keep in mind that the civil war was over 150 years ago and the mood is quite different now. As a US citizen, I couldn't really tell you whether a state seceding would be met with war as in the past or if they would be allowed to quietly leave the union. There is always one state or another where some people think about it, and it seems like these discussions are never met with the hostility that Spain has shown to Catalonia.
I've wondered about this a lot lately. I had family in northern Colorado when this was going on: https://en.wikipedia.org/wiki/Northern_Colorado#Potential_st.... It's relatively common there for people to identify more with the state than the nation, and for people to support the idea of state sovereignty (which is uncomfortable, since the Civil War was catalyzed by slavery, an instance in which every reasonable person would agree that individual sovereignty was already being grossly violated in a way that would justify war if that's what it took). But despite that feeling, many supported a motion to secede from the state and form a new one. It was quite popular within some of my social circles, but then it just fizzled out. Most of the anecdotal accounts of it I've heard indicate it fizzled out just because the governor planned to reject the idea and everyone saw that as a dead-end, so that was that.
