Some trans activists try to silence critics, whom they derisively call TERFs, which stands for trans-exclusionary radical feminists. A spokeswoman for a gay rights group urged a reporter not to “platform” — that is not to quote — those she said held objectionable views, including Martina Navratilova, the retired tennis legend, a champion of liberal and lesbian causes. Ms. Navratilova argues that transgender female athletes possess insurmountable biological advantages.

“So I’m a ‘TERF’ — OK, that’s the way you want to go?” Ms. Navratilova said in response. “I played against taller women, I played against stronger women, and I beat them all. But if I faced the male equivalent of Lia in tennis, that’s biology. I would have had no shot. And I would have been livid.”

Former allies are split so bitterly as to make reconciliation a distant prospect. Half of Ms. Thomas’s University of Pennsylvania team sent a letter to the school, released by a lawyer, saying the swimmer had “an unfair advantage.” Brooke Forde, an Olympic silver medalist with Stanford, however, supported Ms. Thomas. “Social change is always a slow and difficult process, and we rarely get it correct right away,” she stated.