The conflict pits the National Voter Registration Act of 1993 against an Arizona law - the Taxpayers and Citizen Protection Act - enacted by a ballot initiative in 2004. The federal law only requires that people swear they are American citizens under penalty of perjury, the Arizona law requires that actual proof be submitted.
Here's detail from the NYT:
The question for the justices was whether that state law conflicted with the National Voter Registration Act of 1993, which allows voters to register using a federal form that asks, “Are you a citizen of the United States?” Prospective voters must check a box yes or no, and they must sign the form, swearing that they are citizens under penalty of perjury.
Several members of the court’s conservative wing indicated that the state was free to impose additional requirements to make sure only citizens vote.
Justice Antonin Scalia said the federal form was inadequate. “So it’s under oath,” he said. “Big deal. If you’re willing to violate the voting laws, I suppose you’re willing to violate the perjury laws.”
“Under oath,” he added, “is not proof at all. It’s just a statement.”
Patricia A. Millett, a lawyer for several groups challenging the Arizona law, responded that “statements under oath in criminal cases are proof beyond a reasonable doubt” sufficient to lead to the death penalty.
She added that tens of thousands of people had been rejected from the registration rolls because of the Arizona law, though there was no evidence that they were not citizens.
ScotusBlog details the constitutional question presented, and states that generalluy under the Elections Clause Congress has greater power over state laws than under other parts of the Constitution - notable the Supremacy Clause:
Under the Constitution, state legislatures are given the authority to decide “the time, place and manner” of holding elections, including elections for federal offices, but Congress is given the authority to “make or alter such regulations.” The Ninth Circuit interpreted that Elections Clause to give Congress full and final veto power over any state law or regulation dealing with federal elections — that is, for Congress and for the Presidency. Congress used that power in 1993, in enacting the National Voter Registration Act, the Circuit Court said, and any state requirement that conflicts with that Act’s provisions must yield.ScotusBlog also points out that the case is similar to recent ones concerning access to the polls, the effort on the part of some to ensure the integrity of election results and on others to enhance access to the polls. As with other similar cases involving voter id requirements for example, positions on either side of the issue tend to be determined by whether one's political interests are enhanced or inhibited by increased voter turnout.
In reaching the result, the Circuit Court spelled out a special kind of constitutional analysis for judging when a state vote requirement is displaced by a federal one. It ruled that this analysis is different under the Elections Clause than under the Supremacy Clause. Usually, if a state law is found to be preempted, that results from the operation of the Supremacy Clause. But the Ninth Circuit said that, while the Supremacy Clause requires courts to maintain a delicate balance between federal and state interests, the Elections Clause established its own balance with Congress in a position to veto any state procedure on federal elections.
The Circuit Court said that the usual “presumption against preemption” that applies to protect state laws under the Supremacy Clause does not apply when the Elections Clause is at issue. Moreover, it ruled that Congress need not, in overruling a state on federal election procedures, provide a plain statement of its intention to override the state procedure.
