In 2015, the Equal Employment Opportunity Commission, under Mr.Obama, issued a very different ruling, deciding on a vote of three Democrats to two Republicans that discrimination on the basis of sexual orientation was illegal. That ruling, which was reviewed by the Obama administration’s Justice Department, did not formally bind the federal courts, although courts often defer to federal agencies when they interpret laws that come under their jurisdiction.
But in its brief, the Trump administration’s Justice Department said that the E.E.O.C., which has also filed court papers supporting Mr. Zarda, was “not speaking for the United States.”
A lower court on Long Island first considered Mr. Zarda’s case in 2015 and ruled against him, deciding, despite the E.E.O.C. ruling, that sexual orientation was not included in the civil rights law’s prohibition against discrimination based on “sex.” In April, the Second Circuit in New York upheld that court’s decision, even though it noted that there was “a longstanding tension in Title VII case law.”
Federal appeals courts have issued contradictory rulings on the matter. In 2000, while considering the case of a Long Island postal worker, Dwayne Simonton, who was abused at work for being gay, the Second Circuit ruled that the language of Title VII did not bar discrimination based on sexual orientation. The ruling also noted that Congress has repeatedly declined to include such a provision in the law.
“There can be no doubt that the conduct allegedly engaged in by Simonton’s co-workers is morally reprehensible,” the court wrote in 2000. It added, however, that “the law is well-settled in this circuit.”
Shortly after the new brief was filed, civil rights advocates attacked it. In a statement on Wednesday, Vanita Gupta, who ran the Justice Department’s civil-rights division under Mr. Obama, said that the Trump administration’s court filing “contravenes recent court decisions and guidance issued by the Equal Employment Opportunity Commission.”
On Twitter on Wednesday night, Ms. Gupta, who is the president of the Leadership Conference on Civil and Human Rights, noted that only political appointees, not career employees, from her former office at the Justice Department had signed the brief.
The American Civil Liberties Union called the brief a “gratuitous and extraordinary attack on L.G.B.T. people’s civil rights.” In a statement, James Esseks, the director of the organization’s L.G.B.T. and H.I.V. Project, added, “The Sessions-led Justice Department and the Trump administration are actively working to expose people to discrimination.”
In his own statement, Devin O’Malley, a Justice Department spokesman, said that the brief was “consistent with the Justice Department’s longstanding position and the holdings of 10 different courts of appeals.”
Mr. O’Malley added that the filing “reaffirms the department’s fundamental belief that the courts cannot expand the law beyond what Congress has provided.”