The authors of The Federalist Papers neglected to explain the fearsome powers that inhere in the “Dear Colleague” letter under our system of government.

It is the instrument by which middling bureaucrats impose their will on the nation, as the assistant secretary for civil rights at the Department of Education and the principal deputy assistant attorney general for civil rights at the Department of Justice just did in the matter of transgender bathroom policy in our schools and colleges.

The transgender edict is a perfect distillation of the Obama administration’s centralizing reflex, highhanded unilateral rule and burning desire to push the boundaries of cultural change as far as practical in its remaining time in office.

There is no obvious need for a continental nation of more than 300 million people, with more than 14,000 school districts, to have one rule imposed from above on how to handle the sensitive (and suddenly all-consuming) question of which bathrooms transgender students have access to. Communities should be free to formulate their own policies, in accord with their own mores and particular view of what makes sense.

The Obama administration has decided that such localism, allowing for experiment and flexibility, is impermissible under federal law. Its letter is backed by the implicit threat of withdrawal of federal funds (a more appropriate salutation might have been “To Whom It Concerns — Or Else”). The letter contends that Title IX, the federal statute banning sex discrimination in education, mandates its preferred transgender policy.

Yet sex is different than gender identity (as all viewers of “I Am Cait” are supposed to know). If Congress had meant in 1972 — when the current debate was unimaginable — to cover discrimination against the transgendered in the statute, it would have included language to that effect. Or it could have amended the statute at any time. It didn’t. But no matter. Now a letter from a couple of federal mandarins carries as much practical power as a law duly passed by Congress and signed by the president. It is government by epistle.

Whatever one makes of the biology or psychology involved, transgender students deserve to be treated with respect. But it’s not unreasonable to worry about having biologically male students in the same restroom as girls, and vice versa. Somehow, we can assume, schools will figure this out. An obvious compromise is the single-occupancy bathroom, which protects transgender students from harassment and addresses privacy concerns for other students.

This isn’t good enough for the Obama administration. Its diktat effectively requires schools to allow boys identifying as girls into girls’ bathrooms and locker rooms, as well as girls’ dorm rooms and sports teams.

It is infused with a radical spirit. Gender identity is fluid and entirely subjective, the letter makes clear, referring “to an individual’s internal sense of gender.” As soon as a student notifies a school of his or her changing status, it “will begin treating the student consistent with the student’s gender identity.” (It’s easy to imagine scenarios for abuse — if an unscrupulous women’s college basketball team ever wants to topple the dominant UConn program, it should find male players who identify as female for a season.)

The sweep of the measure is symptomatic of the administration’s moral fervor on an issue that was barely on anyone’s radar screen a few years ago. In announcing a lawsuit against the state of North Carolina for an allegedly retrograde bathroom law — i.e., under it, people use facilities matching their birth sex — Attorney General Loretta Lynch compared the state’s action to Jim Crow and resistance to Brown v. Board of Education. She sounded ready to send the 101st Airborne to the bathrooms of Raleigh.

By casting the issue as the next great civil-rights crusade, Lynch and the administration delegitimize the opposition, and prepare the ground for treating traditional beliefs about the immutability of sex as thought crimes. Strong letter, no doubt, to follow.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.