Two men work for the state of Illinois.

One says he is proud to pay his union about $70 each month — money he sees as an investment in his career. The other argues the mandatory fees violate his First Amendment rights.

This conflict is at the heart of a case headed Monday to the Supreme Court, one that labor groups say could deal a catastrophic blow to public-sector unions. In Janus v. AFSCME, the court will consider whether it’s unconstitutional to require government employees to fund unions that represent them.

This is the second time in two years the justices have looked at dismantling what union leaders call a crucial income source, a move that could hamper the labor movement’s power at work and in elections. A month after Antonin Scalia’s sudden death in 2016, the court addressed a similar complaint from a California public teacher and reached a 4-4 split decision, leaving the status quo in place.

This time around, with President Trump’s appointment of Justice Neil M. Gorsuch, the court appears more likely to side with Mark Janus, a child-support specialist for the state of Illinois who asserts that mandatory union fees infringe upon his freedom of speech and association.

The decision would overturn state laws mandating that government workers must pay “agency fees,” which would give some 5 million workers the choice to opt out.

Worker advocates say that reality would detract from a union’s ability to push for safer working conditions and pay. Plus, they add, employees can choose if they'd like to help fund a union's political activities. The mandatory fees cover bargaining efforts.

“This case is yet another example of corporate interests using their power and influence to launch a political attack on working people and rig the rules of the economy in their own favor,” said Lee Saunders, president of the American Federation of State County and Municipal Employees, the union at the center of the case. “When working people are able to join strong unions, they have the strength in numbers they need to fight for the freedoms they deserve, like access to quality health care, retirement security and time off work to care for a loved one.”

Opponents of mandatory fees, however, say the law forces workers to fund groups that back political candidates and causes they oppose.

“I think workers should have the opportunity to go to work and provide a public service without having to fund an organization they don’t agree with,” said Trey Kovacs, a policy analyst at the right-leaning Competitive Enterprise Institute, a Washington think tank.

In Illinois, where the case originated, co-workers at state agencies stand divided over the issue.

Stephen Mittons, a child protection investigator for the Illinois Department of Children and Family Services — the same state government that employs Janus — said he fears the court’s verdict could wipe out an organization he credits with helping him send three kids to college.

The American Federation of State County and Municipal Employees has negotiated to get him regular raises and generous health-care benefits, said Mittons, who is a council member at his local branch in Chicago.

Janus has enjoyed those perks, he added.

“No one has ever been forced to sign a card,” he said. “It is only fair to expect, though, that people who benefit from efforts of the union to contribute to those efforts.”

Janus, however, said his fight should not harm unions if they provide good service to workers.

“If unions were doing such a great job, if the product they have is so magnificent, why do they have to force these fees?” he asked.

The child-support specialist said he grew frustrated when he joined the state government in 2007 and noticed a deduction from his paycheck to cover union fees. He soon discovered there was no way to get his money back.

“The Constitution is a contract with the American people, and that contract says I have right to freedom of speech and association,” he said. “But I’m forced.”

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