The Supreme Court ruled Monday that states cannot on their own require would-be voters to prove they are U.S. citizens before using a federal registration system designed to make signing up easier, AP reports. Justice Scalia, writing for the majority, said a federal law requiring states to “accept and use” a federal form displaced an Arizona law requiring various kinds of proof of citizenship, according to The New York Times. Sen. Ted Cruz responded Monday with an amendment to the Senate immigration bill to counteract the Supreme Court’s decision that would allow states to require IDs before voter registration, as reported by The Hill.