Tina Descovich found herself surrounded by “Muslim dads.” The scene was a school-board meeting late last year in Dearborn, Mich. Local parents were angry about sex-themed books at the school library, which they regarded as “pornography.”

After chatting with Ms. Descovich for a few minutes, a Dearborn dad realized she was a founder of Moms for Liberty, a nonprofit parents’ rights group that came into being on Jan. 1, 2021. He shook his head and told her she didn’t “seem like a racist at all.”

“That’s because I’m not,” she replied.

With its dogged focus on school reform, hostility to teachers unions and opposition to Covid shutdowns and mandates, the group is hated on the left and typecast as far-right—or worse—by much of the media. I speak with Ms. Descovich, a 49-year-old mother of five, at Moms for Liberty’s headquarters here, between Miami and Jacksonville. Seated with her is Tiffany Justice, 44, the group’s co-founder and a mother of four. The modest office has no external signage to identify its occupants. Both women have received such a deluge of threats—by email, voicemail and even handwritten letters—that there’s a deputy at the Brevard County Sheriff’s Office whose main job is to review each one. “Someone calling himself Satan writes to me every week,” Ms. Descovich says wryly. “He lives in Denver.”