“Jeff Sessions’s D.O.J. has made it its mission to oppose, rather than enforce, civil rights law,” said Sharon McGowan, a former Justice Department civil rights lawyer who is the director of strategy for Lambda Legal, which advocates civil rights for lesbian, gay, bisexual and transgender people. “But no matter how many memos he issues, the law is on our side. And so are the courts increasingly.”

Mr. Sessions’s move came three months after the Justice Department, without being asked for its opinion, filed a brief before an appellate court in a private workplace discrimination lawsuit, taking the position that the Civil Rights Act’s ban on sex bias does not cover sexual orientation.

While that move was also greeted with dismay by civil rights advocates, it was less of a reversal than Mr. Sessions’s memo on Thursday, because the Obama administration never took the position that the bar to “sex” discrimination should be interpreted as extending to sexual orientation. Instead, it tried to avoid the question, while welcoming the prospect that the law might “continue to evolve” in that area.

The Supreme Court has not resolved the question of whether “sex” can mean sexual orientation or gender identity. But in a 1989 case, Price Waterhouse v. Hopkins, the Supreme Court ruled that the ban on “sex” discrimination does encompass discrimination against people who fail to conform to gender stereotypes. That case involved a woman who was deemed insufficiently feminine, not a transgender person.

Mr. Sessions’s policy directive was the latest in a series of steps the Justice Department has taken since he became attorney general to curtail the reach of civil rights laws. On his watch, the Civil Rights Division has also changed its position on whether Texas’ strict voter identification law was discriminatory, pulled back from using consent decrees to reform troubled police departments, and began a project to scrutinize affirmative action practices in university admissions.

“It’s striking to see the Justice Department argue again and again to take civil rights protections away from people,” Mr. Esseks said. “When else have they reached out affirmatively, not because they are involved in a case, to say, ‘We think the world is a better place if civil rights laws don’t cover this community?’”

But in a statement, Devin O’Malley, a Justice Department spokesman for civil rights issues, framed Mr. Sessions’s decision in terms of obeying the rule of law.

“The Department of Justice cannot expand the law beyond what Congress has provided,” he said. “Unfortunately, the last administration abandoned that fundamental principle, which necessitated today’s action. This department remains committed to protecting the civil and constitutional rights of all individuals, and will continue to enforce the numerous laws that Congress has enacted that prohibit discrimination on the basis of sexual orientation.”