Do No Harm, a nonprofit that launched last year to oppose diversity initiatives in medicine, has evolved into a significant leader in statehouses seeking to ban gender-affirming care for transgender youths, producing model legislation that an Associated Press analysis found has been used in at least three states.

The nonprofit, not widely known outside conservative medical and political circles, describes itself on its website as a collection of doctors and others uniting to "protect healthcare from a radical, divisive, and discriminatory ideology."

Representatives of Do No Harm turned down...