In a long-anticipated move, the National Labor Relations Board issued a rule Friday that denies teaching and research assistants at private universities legal protection to form unions, retreating from a 2016 decision that cleared the way for collective bargaining at some of the nation’s elite schools.

The proposed regulation asserts that graduate workers are students above all else, not employees of their universities, even if they assist in teaching courses and research that benefits schools. The argument aligns with the position of many of the Ivy League universities that say the work graduate students do is a part of their training and education.

“Make no mistake: Our work is work,” said Rithika Ramamurthy, a doctoral candidate in English at Brown University. “The Trump NLRB’s rule seeks to deny the value of our work by classifying us as students for the sole purpose of diminishing worker power at colleges and universities and in the economy.”

This is the third time the board has reversed its position on the question of graduate worker rights. It supported collective bargaining for graduate students at New York University in 2000, but four years later, with members appointed by President George W. Bush, it reversed the ruling in a case involving Brown. That decision was then overturned in 2016 by a board largely appointed by President Barack Obama.

“This is an area that has been important throughout the time that the board has gone back-and-forth. That’s one of the reasons rulemaking is a good idea,” said Philip A. Miscimarra, a former chairman of the NLRB now in private practice at the law firm Morgan Lewis. “It permits the board to get input from a broad assortment of viewpoints, and a regulation addressing this issue will have greater permanence than what we’ve seen through case-by-case adjudication.”

Graduate student activists expected the 2016 ruling would be overturned as Republicans took control of the labor board. President Trump named Miscimarra, who was the only dissenter in the 2016 graduate ruling, chairman, and nominated lawyers Marvin E. Kaplan, William J. Emanuel and John F. Ring to fill seats on the NLRB. Once Kaplan and Emanuel were confirmed, activists said it was only a matter of time.

Since the labor board’s 2016 ruling, graduate students at 15 private universities across the country have held elections to form unions, according to the National Center for the Study of Collective Bargaining in Higher Education and the Professions at Hunter College in New York. Five private institutions, including New York, American and Tufts universities, have contracts with graduate unions.

Although the legal protection of the board is important, it is not the only path to collective bargaining for graduate students. Teaching and research assistants could also petition their schools for voluntary recognition of their unions, just as graduate students at Georgetown and Brown universities have.

Voluntary agreements spare students from the possibility of the labor board changing position on students’ legal protection as employees, and universities can avoid litigation that tends to arise over the scope of the bargaining unit or other disputes. Many schools, nevertheless, remain reluctant.

Administrators at Boston College and the University of Chicago have refused to recognize graduate student unions, despite pleas from organizers who withdrew petitions with the labor board and instead sought voluntary agreements. University leadership insists that students are not employees.

If the National Labor Relations Board rule is adopted, it could lead graduate students at private institutions to assert the right to collective bargaining under state constitutions and statutes that cover private-sector institutions, said William A. Herbert, the executive director of the collective bargaining center at Hunter.

In May, an appeals court in New York ruled that farmworkers had a right to form unions and collective bargaining under the state constitution, even though they are excluded from the National Labor Relations Act.

“Like farmworkers, if graduate students are carved out by the NLRB’s rulemaking, then it would be logical for them to look to state law,” Herbert said.

Collective bargaining among graduate students is a contentious issue that is as much about economics as it is about power. Graduate students say collective bargaining is the only way universities will listen to their demands for balanced workloads, higher pay and comprehensive health insurance.

“I’ve had to miss bill payments, skip doctor’s appointments, and even work two or three additional jobs to cover my living expenses,” said Yiran Zhang, a graduate worker at Loyola University Chicago. “This is about who has the opportunity to get an advanced degree in this country. This is about who has basic rights as an employee.”