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The U.S. Supreme Court takes up a case Monday that could deal a crippling blow to unions representing millions of the nation's public employees.

The justices are asked to decide whether state government workers who choose not to join a union can nonetheless be required to pay a share of union dues to cover the cost of negotiating contracts.

At stake is the future power and financial health of public sector unions in the 23 states where they have a duty to bargain for both members and non-members alike.

A loss for the unions "would call into question thousands of public-sector contracts covering 9.5 million public employees and affecting scores of critical services, including police, fire, emergency response and, of course, education," said David Frederick, who will argue the case for union teachers from California.

The Supreme Court has long held that requiring non-union members to pay the full amount of union dues would violate their right of free expression, forcing them to subsidize a union's political activities whether they agree with its goals or not.

But since 1977, the high court has said non-union employees can be required to pay a portion of union dues to cover the cost of collective bargaining and prevent "free riders" — workers who get the benefits of a union contract without paying for it.

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Now, in a case brought by non-union teachers in California hoping to get the earlier decision overturned, the justices are asked to decide whether even that requirement violates free expression.

"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 governments should devote to public employees, or what policies schools should adopt to best educate their children," said Michael Carvin, a lawyer for the challengers.

He argues that the government has no legitimate interest in enhancing a union's treasury at a dissenting employee's expense.

As a fallback, the challengers urge the court to rule that teachers should not have to "opt out" of paying the full amount of dues by filling out a form once a year. Instead, teachers should "opt in" if they wish to pay the full amount, they ask.

The union, the California Teachers Association, urges the court not to overturn its earlier ruling, which held that a state's interests in an orderly workplace outweigh the modest interference with employee expression involved in paying a portion of union dues.

The positions the union takes in negotiating contracts involve "bread-and butter employment issues. Collective bargaining does not resemble the wide-ranging, open, and public debate that the First Amendment traditionally protects," said Frederick, the lawyer for the union.

In two recent rulings, the court's conservatives have signaled a desire to overrule the 1977 decision allowing the collection of a portion of union dues.

"I would not feel very good about my prospects if I were the unions, given what we've seen from the Roberts court in the past two years" said Erin Murphy, a Washington, D.C., lawyer who specializes in federal appeals court cases.

A ruling against unions in this case would not lead to a destruction of public sector unions, she said, though they "might not have the same money and the same power."

But Professor David Cole of Georgetown University's law school sees it differently.

"I think public sector unions have long seen this as a real threat to their continued effective existence at a time when unions are on the decline generally," he said.

Supporters of the union position say represented workers generally make more money.

A study by the National Women's Law Center found that women represented by public sector unions are paid 24 percent more and are more likely to have employer-based health insurance. More than half of union-represented workers in the public sector are women.