Transgender athletes’ participation in women’s sports has emerged as a sleeper issue that is poised to attract elusive female suburban voters to former President Donald Trump and other Republican candidates on the ballot.

Some see it as an outlier, but the matter is increasingly drawing the attention of voters as more biological males who identify as women join the rosters of female teams and compete — and win — against girls and women at the middle school, high school and collegiate levels.

“I’m a perfect case in point,” said Kim Jones, a co-founder of the Independent Council on Women’s Sports.

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Ms. Jones had long voted for Democrats, including in the 2020 presidential election. Then her daughter, a student at Yale, was defeated in top collegiate swimming meets by Lia Thomas, a 6-foot-4 biological man who identified as female and was permitted to participate on the University of Pennsylvania women’s swimming team.

“I’m no longer willing to vote for anyone who doesn’t know what a woman is, ever,” said Ms. Jones, a former All-American collegiate athlete. “And I hear from women like me every day. For that matter, also men. It’s a much bigger and more widespread problem than people realize.”

Ms. Jones favors the position held by Mr. Trump, who has pledged to ensure that Title IX, the law protecting girls’ and women’s education at federally funded schools, does not allow biological males to participate in girls’ and women’s sports.

Mr. Trump this week called Democrats, who broadly support allowing biological men to compete with and against women, “ridiculous” and “so far out” of the mainstream.

At campaign rallies, he has promised crowds, “I will keep men out of women’s sports.”

The former president’s views align with polls that consistently show most voters oppose allowing transgender, biological men and boys to participate as females. A Gallup Poll released last year found that nearly 70% of people said transgender athletes should be able to compete only on teams that align with their biological sex. A Los Angeles Times/NORC poll taken in January found that 66% of adults said transgender girls, who are biological boys, should not be allowed to compete in girls’ sports or only in rare cases.

Republicans are betting the issue can help attract female voters as they fight to stem losses to Democrats over the issue of abortion access, where Ms. Harris and other Democratic candidates have dominated.

A New York Times/Siena College Poll released this month found Ms. Harris leading Mr. Trump among female voters by 16 percentage points. Other polls show Ms. Harris with an even wider lead among women.

Jim McLaughlin, who polls for Mr. Trump, said the growing participation of biological males in female athletics is of rising importance, particularly for suburban women, a critical voting bloc.

Mr. Trump has not aired an ad explicitly addressing women’s sports, but his campaign is running ads in battleground states during NFL games that highlight Ms. Harris’ support of policies supporting transgender people, including her past stated support for taxpayer-funded sex change operations for prisoners and illegal immigrants.

“In focus groups and ad testing, it tests off the charts against Democrats,” Mr. McLaughlin said. “It just screams extremist.”

Republicans are spending millions of dollars on advertising for downballot candidates that put a spotlight on transgender participation in women’s sports. The ads are airing against embattled Democrats in key Senate races.

“Our sports are under attack,” a mother warns in a Senate Leadership Fund ad running in Montana, where Sen. Jon Tester, the incumbent Democrat, is trailing in the polls. “Tester voted to let biological men compete against our girls in their sports. Tester even wanted to let men use girls’ bathrooms and locker rooms. It’s disgusting.”

As a presidential candidate, Ms. Harris has remained silent on the issue, and her campaign did not respond to an inquiry about her position.

Her platform promises to protect the rights of transgender people, though she has not indicated where she stands when it comes to ensuring biological males who identify as female can play women’s sports.

As vice president, she participated in the Biden administration’s move in April to revise Title IX to include transgender people on the list of those protected under the law. The new rules did not address sports.

Education Department officials did not set rules on sports participation for transgender athletes. They told advocates they needed more time to work on the issue. Critics said the broad wording opens the door to participation in female sports by biological males.

Riley Gaines, who competed as a collegiate swimmer against Ms. Thomas and now advocates for keeping biological men out of women’s sports, said young female voters increasingly care about the issue.

She said Democrats are dodging the issue because they know the public does not broadly support biological men competing against women, even if they identify as female.

Ms. Gaines, a former University of Kentucky swimmer, tied with Ms. Thomas at the NCAA championships in 2022 and soon became an outspoken critic of transgender participation in women’s sports. She is now director of the Riley Gaines Center at the Leadership Institute and host of “Gaines for Girls” on Outkick.

“What I’m learning is what me and my teammates and girls around the country have gone through — losing out to men on opportunities, or being exploited in locker rooms, or some women being hurt or injured in their sports — that is becoming increasingly common and people are fed up with it,” Ms. Gaines said.