The US supreme court on Tuesday appeared poised to uphold laws banning transgender girls and women from competing in female sports in two conservative states, in a landmark legal battle that could carry profound implications for trans rights across US society.

During oral arguments on two cases of trans students who sued over Republican-supported laws in West Virginia and Idaho that barred them from girls sports, one member of the court’s conservative majority after another voiced skepticism about the students’ cases.

The case had reached the court after West Virginia and Idaho appealed rulings by lower courts, which had ruled in the students’ favor.

Brett Kavanaugh, one of six conservative justices on the nine-member court, suggested the issues at stake in the case potentially threatened the success of women’s sports in the US.

“One of the great successes in America over the last 50 years has been the growth of women and girls sports, and it’s inspiring,” he said.

He cited opinions from groups like the National Collegiate Athletic Association (NCAA) and the US olympic committee suggesting that “allowing transgender women and girls to participate will undermine or reverse that amazing success and will create unfairness”.

The bans against transgender athletes are mirrored in 25 other states.

The laws are aimed at excluding a tiny fraction of the population, with GOP legislators at times unable to identify any trans girls playing sports in their states, and the NCAA president testifying he was aware of fewer than 10 trans college athletes.

Lawyers for the trans students, including the American Civil Liberties Union, argue the bans violate the equal protection clause of the constitution. In the West Virginia case, attorneys argue the ban also violates Title IX, a federal law prohibiting sex discrimination in schools. The states are supported by the Alliance Defending Freedom, a Christian legal group behind major anti-LGBTQ+ cases and anti-abortion efforts.

The arguments centered around Lindsay Hecox, a college student in Idaho, and Becky Pepper-Jackson, a 15-year-old high school student from West Virginia.

Hecox had originally sued Idaho to overturn Idaho’s first-in-the-nation 2020 law categorically banning trans women and girls from women’s sports teams. She later pushed to have the case dismissed, saying she no longer pursued sports and feared harassment. But the supreme court still heard the case.

Her lawyer, Kathleen Hartnett, told the justices that Hecox no longer participated in sports. But she said her client had tried to mitigate any advantage when she was competing by taking testosterone suppressants and estrogen, which she said cancelled Idaho’s justification of its ban.

In a separate hearing for the West Virginia case, the ACLU’s Joshua Block, representing Pepper-Jackson, explained that she had received gender-affirming treatment starting before puberty and argued that this eliminated any competitive advantage.

“By virtue of her medical care, BPJ is already effectively controlled for those sex based advantages, and so she is completely in the position that she would have been if her birth assigned sex had been female, as opposed to a cisgender boy who’s just not very good at sports,” he said, referring to Pepper-Jackson by her initials.

He added: “Virginia’s law treats BPJ differently from other girls on the basis of sex, and it treats her worse in a way that harms her outside the context of athletics.”

In response, Amy Coney Barrett, another conservative justice, asked if transgender girls with no competitive advantage can join girls’ sports, would boys be able to join a girls’ team if they had a similar skill level.

During argument on Idaho, the rightwing justice Clarence Thomas put the question more pungently, asking if there was any difference between what Hecox and Pepper-Jackson were seeking than a “lousy” tennis player who wants to join the women’s team, based on the argument that “there is no way I’m better than the women’s tennis players”.

Idaho’s solicitor general, Alan Hurst, agreed that it was not different.

Addressing Pepper-Jackon’s case, West Virginia’s solicitor general, Michael Williams, called it a “back door attack” on Title IX, a federal civil rights law barring sex-based discrimination in education.

“West Virginia schools can no longer designate teams by looking to biological sex. Instead, schools must place students on sports teams based on their self-identified gender,” he said. But that idea turns Title IX into a law that actually denies those opportunities for girls.

His comments echoed the defenders of similar bans in other states, who argue they are promoting fairness and safety in women’s sports. Trans rights advocates counter the laws are cruel and discriminatory, and that there’s no credible evidence inclusive sports policies have endangered cis girls and women.

The court will consider whether the laws are discriminatory and merit “heightened scrutiny”, a rigorous review where the government has a higher burden to justify the bans. If the court’s conservative supermajority decides the bans don’t warrant heightened scrutiny, it could set a precedent that anti-trans laws are “presumptively constitutional”, the ACLU has warned.

If the court rules trans people are not covered by Title IX, it could boost policies meant to ban trans students’ bathroom access and ability to use chosen pronouns and names, and leave LGBTQ+ youth with fewer protections against harassment, bullying and discrimination.

Pepper-Jackson said in a statement last week that she plays sports to “make friends, have fun, and challenge myself through practice and teamwork”, adding: “All I’ve ever wanted was the same opportunities as my peers. But in 2021, politicians in my state passed a law banning me – the only transgender student athlete in the entire state – from playing as who I really am. This is unfair to me and every transgender kid who just wants the freedom to be themselves.”