Transgender girls competing in Washington K-12 sports will be barred from girls' teams starting Dec. 3 after voters approved Initiative 638 with about 56% support.

Opponents conceded defeat Nov. 5, the Seattle Times reported, calling the result a major setback for trans rights advocates in a state known for its LGBTQ+ protections. The Associated Press called the race Nov. 4. Tens of thousands of ballots remained uncounted, but opponents said the gap was too large to overcome.

The measure directly conflicts with the Washington Interscholastic Activities Association (WIAA) policy that has governed middle and high school sports since 2007.

That policy, the first of its kind in the nation, lets students compete on teams matching their gender identity. The WIAA has said it will comply with the new state law.

For Seattle's nine comprehensive high schools and its private programs governed by the WIAA, the practical impact touches a small number of athletes. The WIAA estimates no more than 10 transgender athletes compete statewide each year out of roughly 225,000 student athletes. Schools are not required to report transgender athletes, and the WIAA has acknowledged the actual number may be higher.

The Seattle School Board, Gov. Bob Ferguson, Sen. Patty Murray, the King County Council, the Washington State PTA and the League of Women Voters all opposed the measure, according to MyNorthwest.

Under I-638's language, all students wanting to play girls' athletics must submit medical documentation verifying their biological sex.

That documentation must come from an examination or provider's confirmation of a student's reproductive anatomy, genetic makeup or testosterone levels. The initiative does not address transgender boys or distinguish between a transgender girl who began a medical transition before puberty and one who has not.

Opponents said the requirement subjects thousands of girls to new verification procedures. Health providers have called the initiative's medical language imprecise.

Libby Watson, campaign manager for No Hate in Washington State, said in a Nov. 4 statement that the results were disappointing but that voters nationwide had rejected candidates who centered trans sports bans in their campaigns.

Brian Heywood, the Redmond hedge fund manager who leads Let's Go Washington, the conservative political committee behind I-638, said in a statement that the vote showed voters believe "Washington's girls are worth protecting and fighting for."

I-638 was the only one of three Let's Go Washington ballot initiatives that passed.

Civil rights groups say a legal challenge is likely. Michele Storms of the ACLU of Washington said in a Nov. 5 statement that the initiative denies transgender students opportunities shared by their peers.

Washington State Democratic Party Chair Shasti Conrad said in a statement the same day that the party considers the initiative unconstitutional and supports organizations planning to challenge it in court.

Legal experts told the Seattle Times before the vote that any challenge would likely hinge on whether I-638 conflicts with the state constitution's Equal Rights Amendment or the Washington Law Against Discrimination.

The state ERA says equality of rights shall not be denied on account of sex. Washington's attorney general would be required to defend the initiative in court.

Under state law, the Legislature cannot modify a voter-approved initiative for two years.

In April, WIAA member schools voted 29-24 on an advisory proposal to limit girls' sports to biological females. The WIAA declined to implement it, saying it conflicted with state law. I-638 changes that legal landscape.

No formal legal challenge had been filed as of Nov. 5.