MIAMI — The day Rodrigo Heng-Lehtinen told his prominent parents about his new gender identity, he did so in a letter that he left on their bed. Then he grabbed a packed bag and, unsure of whether he would be welcomed back, went to a friend’s house to see if his family would love him or leave him.

His shocked parents, Representative Ileana Ros-Lehtinen, a Miami Republican, and Dexter Lehtinen, who served as the top federal prosecutor here, did not hesitate. They grabbed the phone and told him that they loved him and that family trumped all, and asked him to come home. But as with many parents of transgender children, they were also overwhelmed by fear: The future they saw for their then 21-year-old, whom they had named Amanda, would be pockmarked with discrimination and bullying, if not outright violence.

It was this visceral reaction to want to protect her child that drove Ms. Ros-Lehtinen to break from her party’s skepticism or hostility on gay and transgender issues — a stance evident now in North Carolina’s battle over transgender bathroom visits — and become a conspicuous advocate in Congress and more recently in public service announcements. On Monday, Ms. Ros-Lehtinen, her husband and her son, now 30, will appear in the latest one for SAVE, a longtime South Florida gay rights group that hopes to engage the Latino community here.

“I worried about his safety and about his well-being,” Ms. Ros-Lehtinen said, noting that inflammatory debates like the one about school bathrooms serve to further alienate transgender youths and subject them to more bullying and animosity. “I didn’t want him to be depressed. You think of all the parade of horribles that could happen.”