A hotly debated legal and political issue was at the center of today's Supreme Court hearing.
The case challenges unions' right to collect fees from non-members who benefit from union negotiations but who do not want to support the union.
A veteran schoolteacher in Anaheim, Rebecca Friedrichs, is suing the California Teachers Association over the law that requires public-school teachers to financially contribute to unions in order to subsidize collective bargaining efforts.
Justice Anthony Kennedy challenged arguments made by lawyers for the state of California and the California Teachers Association that the current fee system is needed to prevent non-members from becoming "free riders" - workers who get all the benefits of union bargaining and grievance procedures without paying for it.
"The union basically is making these teachers 'compelled riders' for issues on which they strongly disagree," Kennedy said, noting the political nature of bargaining issues like teacher salaries, merit promotions and class size.
Justice Elena Kagan warned that the challengers "come here with a heavy burden" to overturn a nearly 40-year-old case on which thousands of contracts and millions of employees rely.
Justice Stephen Breyer said overturning Abood would require the court to overrule several related cases in which the high court has approved mandatory payments by lawyers to bar associations and mandatory student fees at public universities.
"That's quite a big deal," Breyer said. The case has implications in 22 states.
With files from the Associated Press.
Lynn Rhinehart, General Counsel, AFL-CIO - the umbrella federation for U.S. unions, with 56 unions representing 12.5 million working men and women; Rhinehart co-authored an amicus brief in Friedrichs
Ilya Shapiro, senior fellow in constitutional studies at the libertarian Cato Institute and editor-in-chief of the Cato Supreme Court Review; Shapiro co-authored an amicus brief in Friedrichs. He tweets from @ishapiro