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A federal lawsuit filed by a group of California public school teachers saying mandatory payment of union dues violates their right of free speech is moving forward in the courts..

In essence, in California and 25 other “closed shop” states, the teachers by law must join a union as a condition of employment and pay union dues to keep their jobs.

Union reps say those fees help their efforts to improve workplace safety, for instance, and get better contracts for all employees. They add that teachers can opt-out of paying dues that fund political activities.

But many teachers say opting out is a difficult and intimidating process and claim they face harassment and losing their liability insurance. Others say they get only a fraction of their money back.

So last May, a group of California teachers filed a federal lawsuit, saying the mandatory bankrolling violates their right of free speech.

The case will soon be heard by the 9th Circuit Court of Appeals, on a path to the U.S. Supreme Court.

"Some teachers may want to join the union, that's fine, but the state can't compel membership in the union. That's what this lawsuit is all about," said Terry Pell of the Center for Individual Rights, which is backing the teachers in their court battle.

The lawsuit says the union fees end up bankrolling not just collective bargaining negotiations, but activities many teachers say they fundamentally oppose.

Grade school teacher Rebecca Friedrichs, for example, says she is in favor of school voucher programs. But because of the way the system works, she ends up paying to defeat pro-voucher policies.

“The unions are free to push whatever agenda they please,” says Friedrichs. “I have no problem with that, but I do have a problem with them taking my money to push an agenda with which I do not agree.”

With 325,000 members, the powerful California Teachers Association takes in roughly $190 million a year, with each teacher required to pay about $1,000.

While no one from the CTA would speak on camera, in a statement to Fox News, CTA president Dean Vogel called the lawsuit an attempt "to weaken unions, and the workers they represent."