The court looked to the 1993 National Voter Registration Act, also known as the motor voter law, which was supposed to streamline voter registration across the country. Among other things, it empowered a commission to develop a common registration form that all states have to “accept and use” to populate federal election rolls. The resulting form requires a signed affirmation of citizenship but no supporting documents. Under its 2004 law, though, Arizona rejected completed federal forms without documentation.

The motor voter law’s “accept and use” mandate quite obviously conflicted with Arizona’s requirements. To find differently would require ignoring the law’s straightforward intent: to simplify registration. As Justice Antonin Scalia argued for the majority in a 7 to 2 court decision, if each state could reject federal forms that lack all sorts of extraneous information, why would Congress have bothered to order up a common form?

Since the Constitution gives Congress wide latitude to determine the “time, place and manner” of federal elections, motor voter preempted Arizona’s law.