Child marriage was not an issue of note for Jill Vogel, a state senator in Virginia, until she heard the stories circulating in her district about a man in his early 50s marrying a girl in her midteens, warding off a police investigation of his relationship with her.

Now Ms. Vogel is the lead sponsor of a bill advancing in Virginia’s legislature that would sharply curtail child marriage.

“Our marriage laws in Virginia are not protecting children,” she said.

A similar measure has been introduced in Maryland, and a pending bill in New York goes even further — it would make the state the first to ban marriage altogether for anyone younger than 18.

Worldwide, child marriage is a reality for millions of girls. Though the practice is most common in developing countries, critics who argue it exposes many girls to emotional and even physical harm say it poses a largely unseen threat in the United States as well.