Unlike recent voter-ID cases, Inter Tribal Council doesn't concern what ID must be shown to vote; instead, it concerns what a potential voter must provide the first time he or she registers to vote, which usually happens by mail. The federal form was designed to balance a national interest in convenient voter registration with a competing interest in preventing fraudulent and non-citizen voting. It requires applicants to sign a sworn statement that they are citizens, but requires no additional documentation. States that receive such forms may check their own records to see whether the registrant is a citizen.

Under the NVRA, states may also request that the form's state-by-state instructions tell registrants what additional information their state requires. If the instructions are changed, states can reject forms that do not comply. Arizona's new documentation requirement was passed by the voters in 2004. The state then asked the EAC to amend the form. However, the confirmed members of EAC split 2-2 on Arizona's request. The state could have challenged this inaction in court, but instead it simply put the new requirement into effect.

Because the new requirement wasn't listed on the form, there was no way for Arizona applicants to know that the state needed more than the form asked for. As a result, court documents show, some 20,000 eligible voters -- most native-born and only about one-fifth of them Latino -- were unable to cast ballots.

Scalia applied straightforward statute-reading to conclude that "a state-imposed requirement of evidence of citizenship not required by the Federal Form is 'inconsistent with' the NVRA's mandate that States 'accept and use' the Federal Form." Importantly, his opinion then refused to narrow the scope of Congress's power to supervise federal election procedures in the states.

Scalia noted that Arizona could again petition the EAC to list its new document requirements on the form. Because there are currently no members of the Commission, the request is (shall we say) unlikely to be granted. Arizona, Scalia pointed out, could then request a federal court to order the agency to comply. If that didn't work, it could make a new constitutional challenge to NVRA.

In practical terms, however, Arizona lost big Monday. At oral argument, the Justices had seemed skeptical of the federal form. Scalia himself ridiculed the sworn-statement requirement: "Big deal," he said. "If ... you're willing to violate the voting laws, I suppose you're willing to violate the perjury laws." But Monday the sarcasm was gone; the opinion explicitly deferred to Congress and the Commission.

To salvage its law, Arizona must now insert itself into a multi-year federal Möbius strip of litigation. Even if it wins, the federal form will be amended to explain to potential voters what documentation they must submit, eliminating the trap for the unwary.