U.S. states certainly do have a concept of citizenship! It defines, among other things, who can vote. Now, it's been watered down by the well-intentioned Four

U.S. states certainly do have a concept of citizenship! It defines, among other things, who can vote. Now, it's been watered down by the well-intentioned Fourteenth Amendment, which guarantees citizenship in their state of residence to citizens of the U.S. (the amendment's laudable goal was to prevent the former slave states from denying citizenship to their black residents), but it certainly doesn't have to be that way. I'd very much support an amendment which states that one is a citizen of one's state of birth in the same way that one is a citizen of the United States by birth, and that any citizen of the United States may be naturalised into another state voluntarily and under the laws of that state, such laws being subject to Congressional override. That would enable each state to ensure that its citizens are actually citizens (i.e., with a knowledge of & hopefully respect for its history & traditions), while preventing states from refusing citizenship to a class of citizens and giving the federal government the authority to correct unjust state regulations.