President Obama’s mantra for the past year has been that Congress is broken, so the executive will act. And now, as the stage is set for the new executive, it is dawning on Democrats that living by that sword may mean dying by it. A President can unilaterally revoke prior Presidents’ unilateral actions, and we may soon see just that, in response to Obama’s moves on immigration, climate change, and gun control.

Among the myriad areas subject to upheaval is the President’s administration of Title IX, the 1972 law that prohibits schools that receive federal funding from discriminating “on the basis of sex.” The civil-rights statute has been the primary federal guarantee of equality in educational opportunity for male and female students. This Administration has raised Title IX’s profile by directing schools to take certain actions regarding sexual violence and transgender students, or risk being defunded.

In 1975, the agency that was then the Department of Health, Education, and Welfare said that, consistent with Title IX, schools “may provide separate toilet, locker room, and shower facilities on the basis of sex,” as long as male and female facilities are comparable. The authority to implement this regulation shifted to the Education Department upon its creation, in 1980. The regulation is now at the heart of a case, arising from a dispute between a Virginia school board and a transgender boy, which the Supreme Court agreed to review shortly before the Presidential election. After the election, however, the case’s prospects, along with its broader legal context, appear seriously altered.

Gavin Grimm is a seventeen-year-old high-school senior who transitioned from female to male several years ago, but has not had sex-reassignment surgery. He asked to use the boys’ bathroom, rather than a separate single-stall unisex bathroom, at Gloucester High School, but the county school board refused, and issued a resolution that male and female restrooms and locker rooms “shall be limited to the corresponding biological genders.” In 2015, an official at the Department of Education’s Office for Civil Rights, or O.C.R., responded to the situation with a short opinion letter stating that schools “must treat transgender students consistent with their gender identity” (rather than, say, sex assigned at birth, genitalia, reproductive organs, or chromosomes).

Earlier this year, that letter became the grounds for the Fourth Circuit’s decision in favor of Grimm. Though it was clear that exclusion of each sex from the other’s bathrooms was permissible, the court found, the old regulation was silent on how a transgender person’s “sex” should be determined for this purpose. And while O.C.R.’s current view that “sex” means gender identity was “perhaps not the intuitive one,” it was not plainly wrong. In the face of ambiguity in the regulation’s text, the court applied a legal doctrine that requires judges to defer to the agency’s interpretation of its own regulation. As a result, a letter from an O.C.R. bureaucrat became law.

Three weeks after the court’s decision, an emboldened O.C.R. issued a “Dear Colleague” letter, or guidance, to all schools that receive federal funds, stating that Title IX compliance means allowing transgender students to use bathrooms consistent with their gender identity. Gender identity was defined as a person’s “internal sense of gender.” Thirteen states sued the federal government to challenge this directive, and, in October, a Texas federal court put in place a temporary nationwide injunction to stop O.C.R. from enforcing its view on transgender students and bathrooms.

Now the Supreme Court is set to hear Grimm’s case. The Court is supposed to decide whether O.C.R.’s interpretation of the phrase “on the basis of sex” is entitled to judicial deference. In other words, should the agency’s interpretation carry the day even if it is not one that the Justices think is best? If so, the Court would effectively convert the executive agency’s informally expressed views into the law of the land, because O.C.R. is supposed to be expert in civil rights in education.

The tricky part is that many expect the expert agency’s views to change shortly after January 20th, when Trump’s O.C.R. is installed. (Dear Colleagues: Never mind, we take it back.) Come January, advocates of transgender rights, who have enthusiastically supported judicial deference to O.C.R., will have reason for an extreme pivot, given that the new O.C.R. is unlikely to view “sex” as an “internal sense of gender.” It is awkward now for Grimm’s lawyers to argue zealously for the notion that the agency knows best, when only weeks from now, and in coming years, that doctrine is more likely to harm than to help transgender students. One of the lead attorneys, Joshua Block, of the American Civil Liberties Union, told me that the brief for Grimm to be filed in January will urge the Court to put aside the question of deference to the agency, because even without it transgender students’ right to use bathrooms matching their gender identity “is the only interpretation consistent with the statute,” Title IX. But to the extent that the Court does find ambiguity in the term “sex” in Title IX or the regulation, he said, he will advocate for deference to the Obama Administration’s interpretation.

Anticipated as an opportunity for the Court to consider and perhaps vindicate transgender rights, Grimm’s case is instead poised to illustrate the fragility-in-strength of Obama’s lawmaking-by-letter. On the one hand, such letters have been a powerful tool of this Administration—and have been met by critics with complaints of executive overreach. On the other hand, the recent accomplishments may well be trampled in the changing of the guard: Trump’s O.C.R. can simply issue informal letters retracting prior positions on Title IX that were expressed in other informal letters.

Had Obama’s O.C.R. employed the more onerous procedures, involving public notice and opportunity to comment, that are required to enact a proper legal regulation, its policy would be difficult to dislodge; the new Administration would have to invest in similarly costly and time-consuming procedures to get rid of it. Block noted, “There’s no question that, as a practical matter, regulations protect people more than guidance does.” What’s more, this Administration’s method of threatening to enforce guidance documents as if they were rules is now laid out and ready-made for Trump’s Administration to deploy at will. The President-elect himself has not taken a clear position against transgender access to bathrooms, but his agencies may. One can only hope that his O.C.R. won’t go so far as to take the view that Title IX not only permits but requires schools to treat transgender students according to their sex assigned at birth.

Even before the Supreme Court hears or decides Grimm’s case, Trump’s O.C.R. could issue a new interpretation concerning transgender students. The Court might see such a turnabout as reason to dismiss the case (which leaves the Fourth Circuit’s decision in favor of Grimm in place), or to send it back to the Fourth Circuit to consider in light of the change. But the Court would not have to take either of these actions. And the consequence of a dodge during this term is that the next time a transgender case gets to the Supreme Court, we would likely be talking about deference to Trump’s O.C.R., by a Court with at least one more conservative Justice on it.

If the Court does go ahead and decide the case this term, then the conflicting Trump and Obama interpretations, so close in time, from the same agency, would likely convince the Court to treat O.C.R. as not having a consistent and considered view worthy of deference. Better yet, though, the Court should take the occasion to say that a mere letter, whatever its content, does not merit judicial deference, precisely because it bypasses the process of public input that we should want the executive branch to adopt in forming views on important policies. That point should resonate now more than ever.

If the Supreme Court finds the agency’s letters insufficient, that would leave the Justices to declare now what “on the basis of sex” means, which is really the best shot that transgender students have of securing a federal legal right to access bathrooms corresponding to their gender identity, perhaps for the foreseeable future. After all, we need to know the meaning of the key term, “sex,” in one of the most important civil-rights statutes in the country’s history. That is properly a question for the Supreme Court or Congress, not duelling letters from successive administrators.