The legal dispute over the registration requirement dates back to 2004 when Arizona voters passed a ballot initiative, Proposition 200, designed to stop illegal immigrants from voting. The measure amended state election laws to require voters to show proof of citizenship to register as well as identification to vote at the polls. 

Arizona residents, Indian tribes and civil rights groups sued to challenge the new provisions. 

While the 9th U.S. Circuit Court of Appeals upheld Arizona's right to require voter identification at polling places, the court in April found that the citizenship requirement conflicted with a 1993 federal law designed to make it easier for people to register to vote in federal elections by using a federal registration form.