The U.S. Supreme Court has struck down an Arizona law requiring proof of citizenship before people can register to vote.

In an interesting twist, the majority opinion was authored by Justice Antonin Scalia, typically one of the most conservative voices on the court. Unlike many contentious Supreme Court opinions from the past few years, the decision was not a close one, with seven of the nine justices supporting it.

The case, Arizona v. Intertribal Council of Arizona, centered around the clash between state and federal authority over voting rights. The 1993 National Voter Registration Act allows would-be voters to register through the mail using a "Federal Form," in an effort to increase the voters rolls, turnout and participation in the democratic process.

An Arizona law, however, required applicants to submit proof of citizenship before even attempting to register, and required officials to reject any application that did not include such documentation. The decision does not prevent states to removing people from the voter rolls once there is evidence that they are not eligible to vote.

"Arizona's evidence-of-citizenship requirement, as applied to Federal Form applicants, is pre-empted by the NVRA's mandate that States 'accept and use' the Federal Form," wrote Scalia in the majority opinion. In this case, he said, federal law preempts state law, an interpretation not usually expected from Scalia, a strong advocate for local governance and states' rights.

"Arizona's appeal to the presumption against pre-emption invoked in this Court's Supremacy Clause cases is inapposite. The power the Elections Clause confers is none other than the power to pre-empt. Because Congress, when it acts under this Clause, is always on notice that its legislation will displace some element of a pre-existing legal regime erected by the States, the reasonable assumption is that the text of Elections Clause legislation accurately communicates the scope of Congress's pre-emptive intent," he continued.

In essence, the decision shifts the burden of proof to the state, which must now prove that an applicant should not be allowed to vote, rather than potential voters. Proof of citizenship can be difficult or expensive to obtain, especially in rural areas or for people without reliable transportation. Birth certificates can take months to order and are kept in the state of birth, not the state of residence.

Many would-be voters have maintained that some states illegally purge voter rolls or obstruct voter registration, a practice common in the Jim Crow South that has evolved into discrimination against Americans of Hispanic descent or other minority groups or people with criminal records.

Still, the decision allows Arizona to petition the federal government to include proof of citizenship as a requirement of the Federal Form. It is unlikely that this administration would agree to that, so Arizona could them file a lawsuit and attempt to force the government to do so. No word yet on whether that is something the state intends to do.

But if there is any state with the political will to demand a test of citizenship before voting, it is Arizona.