In a case rife with politics in an election year, the U.S. Supreme Court on Monday will consider a legal challenge to California”s largest teachers union that threatens to shake the foundationof public employee union clout nationwide.

The case centers on an effort by some teachers to essentially defect from the California Teachers Association without having to pay a state-mandated fee to their state and local unions. California is one of 23 states that require such “fair share” fees for public employee unions, even if the public sector worker does not belong to the collective bargaining unit.

The issue is esoteric and mired in legal complexities, but at a time when public employee unions are under political attack in states such as Wisconsin and New Jersey, the California clash is considered a crucial test for organized labor and its political wallet. And California teachers who support their union”s political influence are clearly worried about the outcome in the Supreme Court, where a number of conservative justices in recent years have expressed doubt about allowing states to require fees from reluctant non-union members.

“It would be disastrous for the workers and educators of California,” said Eric Hein, a Pittsburg school teacher who is president of the CTA.

Critics of the status quo say the case boils down to unions milking their members to advance the political causes of organized labor, regardless of whether members, in this case public school teachers, agree with the CTA”s positions. The smaller California Federation of Teachers has also backed the CTA in the Supreme Court, calling the case “an assault” on unions.

Rebecca Friedrichs, an Anaheim school teacher, has led the legal assault on the public employee unions, backed by conservative organizations that have politically attacked organized labor.

“I just want the right for me and all teachers to decide for myself whether to support a union”s negotiating agenda,” Friedrichs said. “Unions are powerful, entrenched organizations more focused on self-preservation than the rights of individual members.”

The challenge to the California union has several twists.

Public employees who do not enlist in their unions already are protected against being forced to pay for overtly political activity, such as lobbying on particular issues, backing political candidates or general union organizing. But since a 1977 U.S. Supreme Court ruling, California and other states have been allowed to collect the “fair share” fees from non-union members to effectively cover the cost of collective bargaining efforts that shape pay, benefits and working conditions.

In California, teachers who choose not to belong to the union kick in about $350 to $400 of the roughly $1,000 state and local union dues under this requirement, according to court papers. The stakes for California public unions and those in other states could be hundreds of millions of dollars a year siphoned from labor”s coffers — one reason 21 states and a host of labor groups have backed the CTA in the Supreme Court.

The unions argue that non-union members should not get a “free ride” from the CTA”s collective bargaining efforts. Hein compares the scenario to four people going out to dinner and three agreeing on the restaurant, then allowing the dissenter to have a meal but decline to pay their share of the tab.

But the other side counters that such fee requirements trample on the free speech rights of teachers who disagree with union positions — they say pushing in collective bargaining for more money and power for teachers from state and local government is inherently political. So the anti-union groups are asking the Supreme Court to overturn its own 1977 precedent in the case, which arose from a challenge involving Detroit area schools.

“In this era of broken municipal budgets and a national crisis in public education, it is difficult to imagine more politically charged issues than how much money local government should devote to public employees, or what policies public schools should adopt to best educate children,” the groups told the Supreme Court.

Labor supporters worry that losing the case in the high court could severely damage public sector unions, which represent far more of the overall public sector workforce than unions do in the private sector. The CTA alone has about 325,000 members, although it is not clear how many teachers opt out of their unions.

Jennifer Thomas, who heads the CTA”s San Jose Unified School District union, estimates less than 1 percent of their unit members opt out of paying union dues. But Thomas, a high school English teacher, considers the case a threat to unions at a time when she believes public school teachers need all the unity they can muster.

“I”m a huge believer in the First Amendment,” she said. “But in the political sphere, you don”t have power if you don”t have money. It”s clear to me this is a systematic attack on public sector unions.”

Howard Mintz covers legal affairs. Contact him at 408-286-0236 or follow him at Twitter.com/hmintz