In a 7-2 decision, the U.S. Supreme Court struck down Arizona's requirement that individuals registering to vote in federal elections must show proof of U.S. citizenship (passport, birth certificate, etc.) because it was pre-empted by federal law which only required people to attest to U.S. Citizenship. The decision, Arizona v. Inter Tribal Council of Arizona, Inc., was not surprising and really a simple matter of pre-emption and statutory interpretation. More here.