Conservative justices on Monday appeared ready to deliver a major blow to public-employee unions by striking down fees paid by nonmembers to cover their share of unions’ collective-bargaining costs.

In Friedrichs v. California Teachers Association, 10 nonunion teachers, represented by Jones Day’s Michael Carvin, urged the high court to overrule a nearly 40-year-old decision that upheld so-called fair-share or agency shop fees that are collected in more than 20 states by public-employee unions required by law to represent all employees in their workplaces. Union supporters rallied outside Monday as the justices heard arguments in the case.

The decades-old decision—Abood v. Detroit Board of Education (1977)—held that the First Amendment concerns raised by compelling objecting members to pay the fees were outweighed by the government-employer’s need for “labor peace”—that is, to avoid having to bargain with representatives with often conflicting demands and to prevent “free riders” who enjoy the benefits of collective bargaining without sharing the costs. The Abood court also ruled that the fees could not be used for political or other activities unrelated to collective bargaining.

During the 80-minute argument in Friedrichs, Carvin told the justices that agency shop fees were an “ongoing deprivation of basic speech and associational rights.” Abood, Carvin argued, is contrary to many of the court’s First Amendment decisions. Overruling it, he said, would make the law consistent.

Carvin’s arguments seemed to strike a sympathetic chord with all of the court’s conservative members, with the exception of Justice Clarence Thomas, who was silent.

Justice Anthony Kennedy was particularly strident in his questioning of Carvin’s opponents—California Solicitor General Edward Dumont, U.S. Solicitor General Donald Verrilli Jr. and the union’s counsel, David Frederick of Washington’s Kellogg, Huber, Hansen, Todd, Evans & Figel.

Kennedy told Dumont, “The union is basically making teachers compelled riders. Many teachers are devoted and think the union is wrong [on critical points]. Agency fees require teachers who disagree to nevertheless subsidize the union on those points.”

Justices Antonin Scalia and Samuel Alito Jr. questioned how it was possible to distinguish between collective bargaining and political activities when, Scalia said, “Everything collectively bargained with the government is in the political sphere.”

Justice Elena Kagan challenged Carvin at length to show a special justification for overruling Abood, a ruling, she said, that “fits pretty well with our speech cases.” Carvin’s argument, Kagan added, seemed to rest on two very recent cases in which some justices criticized Abood but did not overrule it and used the Abood framework to decide them

“It just seems like this is nothing of the kind that we say when a precedent must be overturned,” she said.

Justice Ruth Bader Ginsburg warned that if Abood “falls,” so do decisions upholding fees collected from lawyer-members of mandatory bars and student activity fees at public universities. Carvin countered that those fees are not in jeopardy and can be distinguished. But Kagan interjected, “Those cases start with Abood.”

The court spent little time on a second issue: whether unions should charge fees for unrelated bargaining activities only to nonmembers who affirmatively opt in to those charges instead of requiring objecting members to opt out as is the current practice.

Twenty-five states have so-called “right to work” laws prohibiting employees from being required to pay union fees.