GOP senator probes 18 blue states, DC over Trump's transgender athlete order

His investigation is specifically targeting the inclusion of transgender athletes in girls' and women's sports, and the promotion of policies that allow shared access to facilities, like locker rooms and bathrooms.

The scope of Cassidy's investigation is broad and includes California, Colorado, Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, Washington, and Washington, D.C.

In 19 letters to various heads of state and collegiate education departments, Cassidy alleged that many recipients of federal funding "continue to interchangeably enroll males and females in sports teams that differ from their biological sex," and that the recipients push policies that allow shared access to facilities.

"Under the current and correct interpretation of the law, this is a clear violation of Title IX," Cassidy wrote.

"As Chairman, it is my priority to ensure women and girls have every opportunity to succeed on the field and in the classroom," Cassidy said. "This means ensuring that states receiving federal financial assistance for educational programs comply with federal law and federal agency directives."

Cassidy has given the state education agencies and colleges until Dec. 8 to provide several pieces of information for his investigation, including state and institutional Title IX policies on gender identity, state laws protecting biological females, steps taken to comply with Trump's order, revisions to definitions of sex, policies on athletic participation and facility access by biological males, and complaint records, parental notifications, and disciplinary actions involving students objecting to shared spaces.