On Monday, January 11, the Supreme Court will begin hearing oral arguments in Friedrichs v. California Teachers Association, a case that began working its way through California courts in the spring of 2013. Ostensibly concerned with protecting the free speech rights of public sector workers, Friedrichs’s outcome will in reality decide the viability of public sector unions in the future.

The original plaintiffs are ten California teachers and Christian Educators Association International, an organization of Christian teachers in public and private schools that advertises itself as “an alternative” to the National Education Association, the country’s largest union, and “their affiliates.” The Center for Individual Rights, a law firm devoted to fighting what it characterizes as the “increasingly aggressive and unchecked authority of federal and state governments,” filed the suit on behalf of the teachers and the CEAI to challenge “the constitutionality of California’s ‘agency shop’ law,” which, according to the plaintiffs, violates the First Amendment by requiring non-union public sector workers to nonetheless pay agency fees. Defendants in the case are the NEA and the California Teachers Association, an NEA affiliate.

At the core of the case are “agency fees,” sometimes called “fair share fees.” Agency fees work like this: Public sector unions are required to cover all employees in a given bargaining unit, whether the employees opt into union membership or not. Public sector employees (which include EMTs, firefighters, public school teachers, social workers, and more) thus pay agency fees to their respective unions even if they are not union members, because public sector unions work on behalf of everyone in their bargaining unit, not just union members.

Agency fees do not fund unions’ political activities, but rather strictly the costs of union grievance-handling, organizing, and collective bargaining. In the 1977 case Abood v. Detroit Board of Education, the Supreme Court upheld the right of public sector unions to extract agency fees from public sector workers, and found that agency fees do not violate employees’ freedom of speech, so long as they do not fund unions’ political activities.

But conservative Supreme Court Justice Samuel Alito has long signaled that he would attempt to overturn Abood given the chance. With Friedrichs, his moment may have arrived.