Last year marked a legal turning point for California’s teachers’ unions and public employee unions across the nation. First, Los Angeles Superior Court Judge Rolf M. Treu ruled in June that some of the teachers’ work rules—including tenure, seniority, and dismissal laws—violated the state and federal constitutions. That same month, the U.S. Supreme Court ruled in favor of the National Right to Work Legal Defense Foundation in Harris v. Quinn, holding that home healthcare workers could not be forced to pay agency shop fees to the Service Employees International Union (SEIU).

Treu’s ruling in Vergara v. California inflicted a flesh wound on the teachers’ unions, but Harris sent them reeling. The only way that the Supreme Court’s five-to-four decision could have been worse for the unions is if the justices had decided to broaden it to cover all public employees, not just a subset of them. Instead, Justice Samuel Alito drew a distinction between the home workers and “full-fledged” public employees, who currently must pay dues as delineated in the court’s 1977 Abood v. Detroit Board of Education decision. Nevertheless, Alito’s opinion left the door open for a more expansive court ruling later. He noted that Abood (which holds that the state may force public-sector workers to pay union dues while carving out an exception for the funds that unions spend on political activity) is questionable on several grounds, and went so far as to suggest that collective bargaining issues are inherently political in the public sector. Alito explained, “In the private sector, the line is easier to see. Collective bargaining concerns the union’s dealings with the employer; political advocacy and lobbying are directed at the government. But in the public sector, both collective bargaining and political advocacy and lobbying are directed at the government.” Taking Alito’s reasoning to its logical next step, paying fees to a public-employee union would become voluntary in the 26 states, including California, where it’s now compulsory.

As it happens, a case working its way through the federal appeals process right now from California could be the catalyst for that decision. Friedrichs et al v. CTA pits ten teachers and a union alternative called the Christian Educators Association International against the powerful California Teachers Association. The lawsuit, filed in 2013 by attorneys working with the Center for Individual Rights, takes aim at California’s “agency shop” law, which forces teachers to pay dues for collective bargaining activities, though (per Abood) paying for the unions’ political agenda is not mandatory. The plaintiffs’ lawyers challenging the statute echo Alito’s point out that collective bargaining is inherently political, and therefore all union dues should be voluntary. The Ninth U.S. Circuit Court of Appeals in November issued an order that clears the way for the plaintiffs to petition the Supreme Court. If the justices grant certiorari, a decision could come in 2016.

If the Supreme Court overturns Abood, it would change the political landscape drastically. When Wisconsin’s Act 10 made teacher union membership voluntary, the unions in that state lost about one-third of their membership and a substantial amount of clout. If the same percentage of teachers quit the California Teachers Association, the union would lose approximately $62 million a year in dues. Considering the teachers’ union spent more than $290 million on candidates, ballot measures, and lobbying between 2000 and 2013—by far the most of any political player in the Golden State—such a loss would be crushing. And it’s no secret that CTA spending moves almost exclusively in a leftward direction. Between 2003 and 2012, the union gave $15.7 million to Democratic candidates and just $92,700 to Republicans—a ratio of roughly 99 to one. CTA has also spent millions promoting controversial causes such as same-sex marriage and single-payer healthcare, while opposing voter ID laws and limitations of the government’s power of eminent domain.

And the “fourth co-equal branch of government” wouldn’t be the only teachers’ union to learn what it’s like to live on voluntary contributions. The National Education Association, which hauled in nearly $363 million in forced dues in 2013–2014 and spent about $132 million of it on issue advocacy, would have to curtail its political largess considerably. Like the CTA, the NEA spends almost exclusively on progressive groups and causes. Over the years, the union has lavished gifts on People for the American Way, Media Matters, ACORN, Jesse Jackson’s Rainbow PUSH, and the Center for American Progress. Not surprisingly, the union’s political spending by party is lopsided, too. Between 1989 and 2014, the union directed just 4 percent of its campaign contributions to Republicans, usually backing the least conservative candidate in a primary election fight.

Like most union leaders, recently termed-out NEA president Dennis Van Roekel insists that all teachers should be required to pay the union. “Fair share simply makes sure that all educators share the cost of negotiations for benefits that all educators enjoy, regardless of whether they are association members,” he said in June. Sounds reasonable. But what Van Roekel doesn’t mention is that the unions demand exclusive bargaining rights for all teachers. Teachers in monopoly bargaining states have no choice but to toe the union line. There is nothing “fair” about forcing a worker to pay dues to a union they wouldn’t otherwise join. If Friedrichs is successful and Abood is overturned, it would be a great victory for true freedom of association.

Larry Sand, a retired teacher, is president of the California Teachers Empowerment Network and a contributor to City Journal’s book, The Beholden State: California’s Lost Promise and How to Recapture It.

Originally published in City Journal.