Mark Janus doesn't want to give money to a union. But because the Illinois social worker is covered under a collective bargaining agreement negotiated by the American Federation of State, County and Municipal Employees union, he's legally required to pay a fee to cover the cost of representing him.

Janus is now the named plaintiff in Janus v. AFSCME, a case the Supreme Court is set to hear Monday and that that could fundamentally change the workplace for public employees nationwide. A court ruling against the union, an outcome many believe likely, could seriously dent public unions' coffers by depriving them of a major source of these so-called "fair share" fees.

"It could be devastating to collective bargaining in government, and devastating to all employees who want union representation," said Paula Voos, a labor professor at Rutgers University.

Janus has painted his complaint as a free speech issue. "I'm definitely not anti-union. Unions have their place and many people like them. … I was never given a choice," he told the Washington Free Beacon last fall.

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But for numerous pro-business groups and conservative activists supporting the case, the case represents a chance to undercut organized labor, as one conservative network laid out in a strategy letter. Such a result would both sap public budgets (by reducing what public workers earn) and, recent research suggests, could shift the political field in Republicans' favor.

Federal law already bars unions from using non-members' money for political purposes. It also requires unions to treat all workers in a bargaining unit equally, whether or not they have chosen to join the union. As the National Labor Relations Board states: "Your union has the duty to represent all employees—whether members of the union or not—fairly, in good faith, and without discrimination. ... For example, a union which represents you cannot refuse to process a grievance because you have criticized union officials or because you are not a member of the union."

But for Janus -- and for Illinois Governor Bruce Rauner, who sued the state over fair-share fees in a case that eventually became Janus v. AFSCME -- the law doesn't go far enough. Since these are public workers and negotiate their working conditions with the government, their work is inherently political, Janus' lawyer told HuffPost. The lawyer noted that Janus was concerned that AFSCME was asking for too much in pay and benefits for its workers at a time that the state was having budget issues.

The last time the Supreme Court considered fair-share fees for public workers, in 2016, it split 4-4, upholding the fees' legality but leaving the door open for further challenges. Since then, conservative judge Neil Gorsuch has joined the court as its ninth justice, raising concerns among labor groups that the court is likely to rule in Janus's favor.

Without fair-share fees, many unions could lose a large share of their funding. Across the border from Illinois, AFSCME Iowa Council 61 enjoys an overwhelming 83 percent support among covered workers -- but only 29 percent of those workers are dues-paying members.

"When you provide a service and say 'nobody has to pay,' inevitably some people won't pay, " said Voos, who laid out the economic argument supporting union fees in an amicus brief joined by 35 other economists.

Indeed, if unions are legally required to bargain for paying and non-paying workers equally, it makes economic sense for a worker not to pay. This could create what amounts to a death spiral: When unions lose funding, they have fewer resources to negotiate good benefits for their members; that causes members to leave, which further weakens unions' bargaining power.

The result of weaker unions is often less money for workers -- whether unionized or not. After Wisconsin Gov. Scott Walker restricted public-employee unions in the state, the rate of unionization fell by 6 percentage points and teachers' salaries dropped by an average of $10,000.

There are also political effects. After so-called right-to-work laws are passed, curtailing the fees that unions can collect, "Fewer working-class candidates serve in state legislatures and Congress, and state policy moves in a more conservative direction," a new research paper suggests.

One effect Janus is already having is creating more engaged workers, as, faced with a shrinking revenue stream, unions get more active in explaining what they do.

"We're looking at this as an organizing opportunity," said Mark Cannizzaro, president of the Council of School Supervisors and Administrators, which represents principals in New York City public schools. "Our district meetings are larger than ever — districts that used to have 15 people now have over 100 people showing up."

He added, "To some degree, this court case has given us a reason to wake up and start understanding and talking about the benefits of unions in a way we've never done before."