This file photo shows the Department of Motor Vehicles office in Lawrence on Wednesday, June 27, 2012.

Kansas City, Kansas ? The American Civil Liberties Union urged a judge Thursday to block Kansas from requiring people who register to vote at state motor vehicle offices to show proof of citizenship before they can cast ballots, arguing the mandate violates federal law.

Dale Ho, an ACLU attorney, said that without an injunction thousands of registered voters could be disenfranchised in upcoming state elections. After about four hours of arguments, U.S. District Judge Julie Robinson said she would take the matter under advisement.

Kansas Secretary of State Kris Kobach said that if the temporary injunction were granted, it would create a two-tier system. People who register to vote at motor vehicle offices would be able to vote in federal elections, but not state ones, while those who registered somewhere else without showing adequate proof wouldn’t be allowed to vote in either, he said.

“State law is still in place,” said Kobach, who helped craft the state’s proof-of-citizenship requirement. “They can’t claim the (national law) trumps state law for state elections.”

The ACLU is representing six Kansans who registered to vote when they obtained their driver’s licenses but were not allowed to cast ballots because they did not comply with a state law requiring they prove their citizenship.

Ho argued Kansas’ proof-of-citizenship law, which went into effect in 2013, violates the National Voter Registration Act of 1993. In a lawsuit filed in February, the ACLU argues that Congress specifically rejected a provision that would’ve required proof of citizenship or other documentation when people register to vote.

“The integrity of the process is threatened when 16,000 people are prevented from voting,” he said.

Kobach has long supported measures in Kansas and elsewhere requiring documents such as a birth certificate, passport or naturalization papers as a way to prevent noncitizens — particularly those living in the U.S. illegally — from voting.

Critics say incidents of noncitizens registering to vote are extremely rare. They argue such Republican-backed laws hurt voter registration efforts and disenfranchise voters that tend to support Democrat, including minorities and college students who may not have the documentation readily available.

Kansas for years has been at the forefront of efforts for more stringent registration laws and has been embroiled in several legal fights over the issues. Ho has described Kansas as the nation’s “epicenter of voter suppression.”

Also in February, voting rights groups sued a top federal elections official who decided that residents of Kansas, Alabama and Georgia could no longer register to vote using a federal form without proof of citizenship. Kobach has intervened in that lawsuit in support of the move.