WASHINGTON (AP) — The Supreme Court will consider limiting the power of government employee unions to collect fees from non-members in a case that labor officials say could threaten membership and further weaken union clout.

The justices said Tuesday they will hear an appeal from a group of California teachers who say it violates their First Amendment rights to have to pay any fees if they disagree with a union’s positions and don’t want to join it.

The teachers want the court to overturn a 38-year-old legal precedent that said unions can require non-members to pay for bargaining costs as long as the fees don’t go toward political purposes. Public workers in half the states currently are required to pay “fair share” fees if they are represented by a union, even if they are not members.

But the high court has raised doubts about the viability of that regime in two cases over the past four years. The court has stopped short of overturning the 1977 case, Abood v. Detroit Board of Education case, but in a 5-4 opinion last year, Justice Samuel Alito called Abood “questionable on several grounds.”

Alito said a “bedrock principle” of the First Amendment is that “no person in this country may be compelled to subsidize speech by a third party that he or she does not wish to support.”

The lead plaintiff in the case is Rebecca Friedrichs, a public school teacher in Orange County, California, who says she resigned from the California Teachers Association because it takes positions that “are not in the best interests of me or my community.” She says she is still required to pay the union about $650 a year to cover bargaining costs.

The union says the fees are necessary because it has a legal duty to represent all teachers at the bargaining table, even those who are not part of the union.

A federal district court ruled against her and the other challengers, saying the outcome was clear under Abood. The 9th U.S. Circuit Court of Appeals affirmed.

Leaders of some of the nation’s largest public sector unions issued a joint statement calling the lawsuit an effort to weaken labor rights.

“The Supreme Court is revisiting decisions that have made it possible for people to stick together for a voice at work and in their communities — decisions that have stood for more than 35 years,” said the statement from the National Education Association, American Federation of Teachers, California Teachers Association, American Federation of State, County and Municipal Employees and Service Employees International Union.

The Center for Individual Rights, a conservative group working with the plaintiffs, argues that even basic union goals such as negotiating pay raises and boosting school budgets can clash with the political and educational beliefs of many teachers.

“We are seeking the end of compulsory union dues across the nation on the basis of the free speech rights guaranteed by the First Amendment,” said Terry Pell, the group’s president.

The Supreme Court’s rationale in 1977 for allowing the fees was to help promote labor peace and prevent non-members from “free riding,” since the union has a legal duty to represent all workers.

A ruling in favor of the teachers challenging the fees could sap finances at all unions representing teachers, firefighters and other government workers, labor leaders and other experts say.

“When unions are required to provide representation, if people don’t have to pay for that, a lot of them are going to opt for that free option and that’s going to cause enormous problems for the viability of unions,” said Benjamin Sachs, a professor at Harvard Law School specializing in labor law.

As private sector union membership has steadily declined over the past four decades, unions representing government workers have emerged as a powerful force in organized labor. But they have come under increasing attack as officials in Wisconsin and other states blame them for generous pension and benefit packages that cash-strapped governments no longer can afford.

Public-sector workers have a union membership rate of 35.7 percent, more than five times higher than that of private-sector workers at 6.6 percent, according to the U.S. Bureau of Labor Statistics.

The case, Friedrichs v. California Teachers Association, 14-915, will be argued when the Supreme Court begins its new term this fall.