Presumably, if they decided not to accept US passports as proof of citizenship at all then they would require either a US birth certificate or the like or a naturalization certificate. This would mean that people who naturalized as minors through their parents and don't have naturalization certificates would need to spend $460 getting one. I'm sure any decent court would view this financial discrimination against a subset of citizens as unconstitutional too.It's interesting. I think the States can decide for themselves, as long as they follow the Constitution (and therefore without discriminating among US citizens).