In a surprise decision, the Supreme Court today ruled that an Arizona state law may not require documentary proof of citizenship from voters. In tossing out this requirement, SCOTUS concluded that this is at odds with federal law — the National Voter Registration Act of 1993 — that allows voters to register by swearing on a federal form under penalty of perjury that they are citizens.

This could have sweeping ramifications for the battle over voting rights that is underway in other states. But there is a major caveat.

The SCOTUS decision, which was authored by Justice Antonin Scalia, finds:

We conclude that the fairest reading of the statute is 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.

SCOTUS is upholding a previous ruling by an appeals court that the state and federal requirements “do not operate harmoniously.” In contrast to the federal law, the Arizona law had required further proof of citizenship. Today SCOTUS reaffirmed that the federal law is paramount, and that state law must comport with it.

At the same time, though, the decision also says Arizona can ask the federal Election Assistance Commission to change the federal registration requirement. If that doesn’t happen, Arizona can mount a legal challenge to the EAC’s refusal to do so — which means there are still legal avenues for Arizona to pursue.

Still, today’s decision could have implications for around four other states that have similar laws, and another dozen or so that are contemplating similar ones. But it could have even broader ramifications than this for the future of the ongoing battle over voting rights in general.

Wendy Weiser, a voting rights expert for the Brennan Center for Justice, tells me: “This is significant. It’s an affirmation of Congress’s critical role in regulating voting and having minimal national standards at a time when many Americans are hoping Congress is going to step in and impose further national standards.”

In his victory speech, Obama vowed to do something about the long lines and other severe voting problems that persisted on Election Day 2012. He has established, via executive order, a commission to come up with a series of solutions to them. Among the solutions experts hope will be suggested: Federally mandated early voting periods; federally required modernization of registration at the state level; and national standards for voting machine access per capita. These could run into a roadblock of opposition among Republicans in Congress, but theoretically, today’s SCOTUS ruling could bode well for such efforts.

“The decision was a ringing endorsement of Congress’ authority to regulate in the area of voting, at least where federal elections are concerned,” Weiser says.

Of course, the continuing battle over voting rights is very broad, and today’s SCOTUS decision only touches one legal technique those who would restrict voting are employing. The Brennan Center has compiled a list of currently pending state level initiatives all over the country that would make it harder to vote, including ones that would move up the voter registration deadline, limit early voting, require photo IDs to vote, eliminates Election Day registration to vote, and so on. And today’s decision has no bearing on the challenge to the core of the Voting Rights Act, which is expected to go before SCOTUS soon.

Today’s SCOTUS decision impacts “only one of multiple different kinds of ways that states in recent years have sought to make access to voting more difficult,” Weiser says. But, she adds, the decision “strongly reaffirmed Congress’ power to regulate federal elections. And that’s critical.”