The ACLU of Montana filed a lawsuit Tuesday challenging the constitutionality of a proposed ballot initiative that would require transgender residents to use public bathrooms and locker rooms that correspond with their sex at birth.

The lawsuit was filed in District Court in Cascade County on behalf of seven transgender Montanans, the parents of a transgender 9-year-old and the city of Missoula. The Bozeman City Commission voted Monday to join the effort.

"This proposed measure legalizes discrimination," said Alex Rate, legal director for the ACLU of Montana.

The ACLU and the plaintiffs argue the Locker Room Privacy Act would deprive transgender Montanans of equal protection under the law and violate their rights to privacy, dignity and due process.

The lawsuit asks the court to declare the initiative unconstitutional and to prevent Secretary of State Corey Stapleton from placing it on the November 2018 ballot.

The Montana Family Foundation is sponsoring the initiative. Foundation president Jeff Lazloffy has argued that predators claim they are transgender to access public bathrooms used by the opposite sex.

"High school girls shouldn't be forced to shower in front of a boy, even if he does think he's a girl," Lazloffy said in a statement Tuesday. "Boys shouldn't have to change clothes in front of a girl, even if she thinks she's a boy. It's just common sense."

Lazloffy said Initiative 183 offers solutions such as single-stall changing facilities.

While his arguments center on locker room use, plaintiffs focused on the initiative as it would apply to public restrooms.

"This morning, I walked down the hall and used the women's restroom," transgender plaintiff Roberta Zenker told those gathered at the Capitol Rotunda. "It was not lost on me that if I-183 passes, I would not be able to use that restroom."

The law would force her to use the men's restroom and face possible harassment, humiliation and embarrassment or risk breaking the law by using the women's restroom, she said.

"What better way to discriminate against a class of people than to effectively exclude them from public places," said Zenker, who transitioned to a woman 11 years ago.

Lazloffy argued the lawsuit is premature because the initiative has not yet qualified for the ballot. Supporters have until June to gather the nearly 26,000 signatures needed.

Rate noted that in an earlier case challenging the constitutionality of a ballot measure, former Montana Supreme Court Justice Jim Nelson wrote that placing a facially invalid measure on the ballot would be a waste of time and money for all involved.

The high court ruled last month that the ballot language approved for the initiative needed to be re-written because it did not include the initiative's definition of sex and was otherwise vague.

Rate said he hoped the judge would rule before the November 2018 election.