The suit, joined by the Washington Education Association and the advocacy group TransFamilies, argues that the initiative violates Washington's Equal Rights Amendment by imposing different eligibility steps on girl athletes than on boy athletes. The ACLU also contends that I-638 runs afoul of state constitutional procedural requirements for laws that amend existing statutes, because it changes the Washington Law Against Discrimination and other anti-discrimination laws that explicitly protect transgender people.

"I-638 also violates state constitutional procedural requirements for enacting new laws that amend existing laws," the ACLU said in a statement announcing the filing. "It violates this by amending the Washington Law Against Discrimination and other anti-discrimination laws that explicitly protect transgender people."

The initiative was approved by voters last week. It requires that a child's sex be verified before participation in school sports, a process the plaintiffs say creates a separate and unequal burden for girls.

Lets Go Washington, the group backed by hedge fund manager Brian Heywood that advanced two other initiatives this election cycle, defended the measure's sex-based eligibility rules. In a statement, the organization said the U.S. Supreme Court has allowed a carve-out for "sex-based eligibility restrictions" and described how verification could work in practice.

"Males undergo hernia checks during their routine sports physical, and a simple absence of this exam is enough for a doctor to confirm that their patient is a female," Lets Go Washington said.

The legal challenge is the latest flashpoint in a broader national debate over transgender athletes' participation in school sports, and it sets up a confrontation over how Washington's constitution treats sex-based classifications and the rights of transgender students. For families across the state, the lawsuit means the new requirements — and the question of who can play — remain unsettled as the case moves forward.