Kansas Secretary of State Kris Kobach (John Hanna/ AP) Kansas Secretary of State Kris Kobach (John Hanna/ AP)

A federal judge on Wednesday sided with two states that want to force new voters to prove they are citizens, over a federal elections commission. In the process, the ruling opens the door to other states that want to impose proof-of-citizenship requirements — and an almost certain Supreme Court showdown over the latest front in the war on voting rights.

Kansas and Arizona both require new registrants to provide a birth certificate, passport or some other proof that they are citizens; they sued the U.S. Election Assistance Commission after the EAC refused to modify its federal form to account for the state requirements. On Wednesday, U.S. District Court Judge Eric Melgren ruled the EAC didn’t have the authority to deny Kansas and Arizona’s request, and ordered the EAC to modify a national voter registration form to include special instructions for residents of the two states.

The ruling won’t have much of an immediate impact on voters. Few voters actually register using the federal form; elections officials in Maricopa County, population nearly 4 million, estimated only about 900 residents had registered to vote using the federal form without showing proof of citizenship.

And even those 900 voters, and the rest in both states who registered using the federal form without showing proof of citizenship, would be allowed to cast ballots in federal elections, under a 2013 Supreme Court decision known as Arizona v. Inter-Tribal Council of Arizona, Inc. That ruling, authored by Justice Antonin Scalia, held that Arizona had to accept the federal voter registration form without proof of citizenship.

But it also held that states have the power to set voter qualifications. Arizona Attorney General Tom Horne took that to mean voters who registered using the federal form and didn’t show proof of citizenship could cast ballots in federal elections, but not state elections (See our October story here).

The long-term impact of Melgren’s decision means that groups that conduct voter registration drives have new barriers to overcome in states that have proof-of-citizenship requirements. Most of those groups, like the League of Women Voters and the NAACP, use the federal form to sign up new voters — it’s simpler, and it can sign up a voter in Maine as easily as someone from California.

The case, though, is a preview of a larger battle to come, one that’s almost certain to reach the Supreme Court. Wednesday’s ruling didn’t address the constitutional question behind the states’ lawsuit: Kansas and Arizona argued that Congress doesn’t have the authority to regulate what information states collect to verify a new registrant is eligible to cast a ballot. The Supreme Court didn’t answer that question directly in the Inter-Tribal opinion.

If the EAC appeals the ruling all the way to the Supreme Court, the justices will have the opportunity to take up just how federal and state governments divide responsibility for election rules, and whether the Constitution gives the federal government the authority to supersede stricter — or less strict — state laws.

One thing is certain: A federal judge just gave two states the go-ahead to place a higher burden on voters, through proof-of-residency tests, than the federal government does. That’s been a hallmark of laws and proposals that have advanced through Republican-led legislatures in recent years, and it’s likely more states will raise their standards for new voters in coming years. Already, Georgia has said it will ask the EAC to craft a state-specific change in the national voter registration form.

Until, at least, the Supreme Court jumps on the caes.