HELENA, Mont. (AP) — The American Civil Liberties Union, the ACLU of Montana and the Native American Rights Fund are challenging a Montana law that they say severely restricts Native Americans' ability to vote.

The groups filed a lawsuit in District Court in Billings Thursday arguing a voter-approved referendum that restricts who can collect ballots, and how many they can return to county election offices, disproportionately burdens Native Americans who live in rural areas without home mail service.

The Ballot Interference Prevention Act "ignores the everyday realities that face Native American communities,” said Jacqueline De León of the Native American Rights Fund. “It is not reasonable to expect voters to drive an hour to drop off their ballot, so collecting ballots in reservation communities just makes sense.”

Rural Native Americans also have difficulty getting to polling sites due to limited hours, unreliable roads, lack of access to a vehicle and limited money to buy gas, the lawsuit states.

Western Native Voice and Montana Native Vote work to promote voter participation in the Native American community. On average, they collect over 85 ballots per organizer, the lawsuit states. Under BIPA, workers could only collect six ballots to return to election offices.

“A main tenet of our organization’s principles is to encourage civic engagement,” said Marci McLean, executive director of Western Native Voice. “We developed a robust get-out-the-vote program and coupled it with an official ballot collection program. We have organizers on every reservation in the state, and in urban areas. For Native voters living on a reservation, this law directly harms our ability to participate in our democracy.”

The lawsuit asks the judge to block the law, which voters passed with 63% of the vote in November 2018. The lawsuit notes the ballot language did not say people would be limited to collecting six ballots.

The lawsuit names Secretary of State Cory Stapleton and Commissioner of Political Practices Jeff Mangan as defendants in their official capacities. Stapleton's office did not immediately respond to a request for comment. Mangan said his office had not seen the lawsuit and would release a statement “upon review and when appropriate.”

All seven Montana Indian tribes are listed as plaintiffs.

Election administrators in three Montana counties told a state legislative committee last month that BIPA is frustrating electors, suppressing votes and is easily bypassed because people could collect absentee ballots and mail them rather than drop them off. People who drop off another voter's ballot at an election office are asked to fill out a form listing their relationship to the voter.

The way the law is written, it's unclear if it prohibits collecting ballots and mailing them, said Lillian Alvernaz, Indigenous justice legal fellow with the ACLU of Montana.

“The lack of clarity leads to a very legitimate concern among get-out-the-vote organizers that they could be prosecuted for their work,” Alvernaz said. "Furthermore, even if it was clear that organizers could indeed mail ballots on behalf of other individuals, BIPA would still harm Native people living on rural reservations because due to poor mail services in many communities, ballots have historically been collected and hand-delivered to a polling place.”