TRENTON -- New Jersey's public labor union leaders say they are intently watching a case before the U.S. Supreme Court on Monday that could strike down compulsory membership dues and threaten how they are funded.

The outcome is of major consequence in the Garden State and about 20 other states where public workers are required to join their union or participate in a so-called fair-share arrangement, where they pay only the portion of annual dues that supports the union's nonpartisan activities, like contract negotiations.

New Jersey has about 344,000 public union members.

In Friedrichs v. California Teachers Association, a group of teachers argue the arrangement violates their First Amendment rights to free speech. A ruling in their favor would overrule a nearly 40-year-old Supreme Court decision that allowed these fair-share, agency fee arrangements so all employees pay for the benefits they receive from collective bargaining. Oral arguments will be held Monday in Washington.

"All eyes will be on the Supreme Court this spring, when we learn if the conservative-packed court will try to cripple public sector collective bargaining by ruling that nonmembers will not have to make a contribution toward the cost of their representation, thus forcing union members to unfairly pay the cost of free riders," said Hetty Rosenstein, state director of the Communications Workers of America.

"But however the decision comes out, we will do what we always do: fight for the rights of all workers on the job and for the critically important services that our members provide."

The union's dues do not "in any way" pay the cost of partisan politics, she said.

The lead plaintiff in the case, Rebecca Friedrichs, an elementary school teacher for 28 years, argued workers should also be able to opt out of union's less overt political activities. She said she does not want to subsidize a negotiating agenda with which she disagrees.

"In my view, every individual has the right to choose the organizations that advocate on their behalf, and that includes public employee unions," she said in a recent conference call with reporters. "I admire the history of unions and the spirit in which they were born... But in recent years, unions have become what they used to fight. They're powerful, entrenched organizations more focused on self-preservation than on protecting the rights of individual members."

Her union was bargaining for higher salaries and tenure laws that she said made her job more challenging.

These disagreements over salaries and benefits amount to fundamental political disagreements over the best use of tax dollars, said Terence Pell, president of nonprofit representing Friedrich, The Center for Individual Rights.

"We recognize that many public employees agree with their union and want to support it. But the fact is that some do not. As it stands, tens of thousands of public employees are being denied their basic right to free speech and association by being compelled to financially support unions with which they fundamentally disagree, on everything from tenure and salaries to pensions and politics," he said on a call with reporters.

Despite the longstanding precedent, Pell argued that it has become more difficult to distinguish between unions' nonpolitical and political activities since the 1977 ruling that said agency fees or fair-share agreements are constitutional.

"Unions have become more and more political, and it's become more and more clear that even the things they negotiate for during collective bargaining have a direct connection to political disputes," he said.

But the legal challenge is not an effort to dismantle unions but an effort to establish that collective bargaining is political speech, he said, adding "A win in Friedrichs will mean that unions will have to compete for dues just like any other professional organization, which we feel is good for unions and ultimately for the members they serve."

Patrick Colligan, president of the New Jersey State Policemen's Benevolent Association, said the benefits of membership, such as legal protection, give him confidence a ruling against mandatory membership dues wouldn't devastate the organization. He noted that if unions lose, they'll have to market themselves to potential members.

"A lot of members that aren't active in unions don't see what their unions are doing," he said, or "they may not all be in love with their unions, but they need to understand their benefits and compensation are very tied to what the union does. No matter what union you're in, you're deriving some benefits from it."

In New Jersey, fair-share representation fees for non-members cannot exceed 85 percent of the full-freight membership fee. In 2012, just 2,041 employees opted for the New Jersey Education Association's representation arrangement, while the public union -- the state's largest -- had about 195,000 members, according to PolitiFact New Jersey. Nonmembers paid 81.8 percent of the NJEA dues that year.

"We believe that it's just a fundamental principe of fairness. You should pay the cost of the representation you receive," NJEA spokesman Steve Baker said. "We maintain, and public sector unions maintain, that you should not get those benefits without paying for the portion of the union dues that go for paying for those benefits."

Samantha Marcus may be reached at smarcus@njadvancemedia.com. Follow her on Twitter @samanthamarcus. Find NJ.com Politics on Facebook.