Don't be fooled by the U.S. local governments' threats not to cooperate with the federal government. In the U.S., the federal immigration office and the state sheriff department can enter and operate in the cities and local governments to implement their responsibilities. That is the same for, say, for the law enforcement branches of agencies such as U.S. Post Office and even I.R.S., an federal agency. Therefore, even the state and local government does not cooperate, the federal immigration operation will not be a problem. Also, federal government can always make local governments operate with it by giving or disallowing federal grants to the localities. Then, the local government may not cooperate under one law but also cooperate under another law. The cities or states passing laws to be shelter state or city just one side of the story.