Civil rights groups have filed a lawsuit in federal court claiming that a century-old Louisiana law discriminates against foreign-born, naturalized U.S. citizens by arbitrarily subjecting them to “heightened” voter registration requirements that don’t apply to native-born citizens.

Attorneys from the Southern Poverty Law Center and the Fair Elections Legal Network are seeking a court order blocking enforcement of the state law, which has been on the books since 1874.

Their class-action suit claims the law is unconstitutional because it requires naturalized citizens to provide documents proving their citizenship when they register to vote, while other residents simply must swear that they are citizens on the voter registration application.

Many residents won’t be able to vote in November’s presidential and congressional elections if state and local officials continue to enforce the law, the suit alleges.

The suit says Louisiana’s law appears to be the last of its kind still enforced in the U.S. Similar laws in other states used to be more common but have been struck down as discriminatory, the suit adds.

Four other states — Alabama, Arizona, Georgia and Kansas — currently have proof-of-citizenship requirements for voter registration. Jon Sherman, an attorney for the Fair Elections Legal Network, said Louisiana’s law is different from the others because its requirements single out naturalized citizens.

Louisiana Secretary of State Tom Schedler and state Commissioner of Elections Angie Rogers are named as defendants in the suit, filed in the Middle District of Louisiana.

The named plaintiffs include Norma Flores, a 51-year-old resident of Jefferson Parish who was born in Honduras and became a U.S. citizen in 2008. The suit says the law stymied Flores’ efforts to register and vote in the 2012 presidential election, the 2015 gubernatorial election and 2016 presidential primary.

“Flores and her U.S.-born son filled out voter registration forms together, mailing in their forms at the same time in different envelopes,” plaintiffs’ attorneys wrote. “A few weeks later, Flores’s son received a confirmation of voter registration while Flores received nothing.”

Flores has made three attempts to register. She received nothing in response to her first two attempts; after her third attempt, she received a letter that said she had 10 days to prove her citizenship, according to plaintiffs’ lawyers.

“The 10-day window may have passed before she even received the letter, but once she received it, she was preparing for a trip and was unable to go to the Registrar’s office within 10 days of receiving the letter,” they wrote.

Phil Trupiano, chief deputy for Jefferson Parish Registrar of Voters Dennis DiMarco, said his office has consistently enforced the requirement for foreign-born residents during his 14-year tenure.

“Nobody complains about it,” he said. “If they have the proper paperwork, it’s no big deal. If they don’t have the proper paperwork, they have to get it.”

DiMarco said his office sent at least two letters to Flores in response to her applications — one in August 2015 and another in early 2016.

“We didn’t ignore her. We wouldn’t ignore anyone,” he said.

Proof of citizenship includes a certificate of naturalization or a U.S. passport.

The suit says nearly 182,000 foreign-born people were living in Louisiana as of fiscal year 2014, and 72,250 of them were naturalized U.S. citizens.

“Many naturalized voters are hindered in complying with this discriminatory law by language barriers, unfamiliarity with the registration process, transportation issues, tight deadlines and work and family obligations,” the suit says.