If an urban center were to challenge a state or federal authority, that city would simply be forced into compliance by military/police means. It's terribly simple to manufacturer a premise for using National Guard.

Then, if your city did decide to go to war with the state/feds, and if they did manage to hold their own against the assault, it's still a city. That means, by definition, it contains more humans than the land mass upon which it sits can renewably support. That requires importing everything from eggs to flour to petroleum to baby diapers. You think for a second that the state/feds would permit those imports to go through? Of course not. It would become a siege.

Best of luck with your urban revolution.