Anybody “could” do anything. It’s all made up. Government is just a set of rules we all agree to play by, so any group of people could “decide” to stop playing by the rules and start playing by different rules. That’s why the Confederacy happened.

The only question is, do you have the strength (militarily, usually) to back it up. Because, no, legally what you’re talking about isn’t a thing.

Not that there’s an alternative, of course, if no agreement is reached. The government just stops. Taxes continue, the military goes on (at least for two years), but that’s about it. There’s no default state or last good save for us to revert back to.