The case began as a challenge to Proposition 200, an initiative passed by Arizona voters in 2004 that requires voters to prove that they are U.S. citizens by showing a birth certificate, passport, driver’s license, naturalization certificate, or tribal document before registering and also to show identification when casting their ballots. Under the law, state election officials must reject any voter registration forms not accompanied by sufficient evidence of citizenship—even the federal form produced by Congress under the National Voter Registration Act (NVRA), which doesn’t generally require the documents Arizona does.