Mark Janus has worked for years as an Illinois state employee, and pays about $550 annually to the powerful public-sector union known as AFSCME.

While not a member of the union, he is required under state law to hand over a weekly portion of his paycheck – which he says is a violation of his constitutional rights.

"I work for Health and Family Services, and I'm forced to pay money to a union that then supports political causes that I don't agree with," Janus told Fox News.

Now, Janus' free-speech fight is before the Supreme Court, which holds arguments in the appeal on Monday. And the political and financial stakes are huge for the broader American labor union movement, which already has begun sounding the alarm about the consequences should the justices rule for Janus.

'I just look at it as an average guy just standing up for his own rights of free speech.' — Mark Janus

"Unions would lose resources, contracts would become weaker, and the membership would become divided," said John Scearcy, secretary-treasurer of Teamsters Local 117, representing 16,000 workers in Washington state. "There is a strong likelihood that your voice as a public sector union member could be significantly weakened."

The high court is being asked to overturn its four-decade-old ruling over so-called "fair share" fees, allowing states to require government employees to pay money supporting collective bargaining and other union activities – whether they join the union or not.

While the current case applies only to state employees, the repercussions could affect unions nationwide.

The Supreme Court had deadlocked when the issue was revisited two years ago, just after Justice Antonin Scalia died suddenly.

His Trump-picked replacement, however, is expected to be the deciding vote this time around.

Justice Neil Gorsuch faced strong labor union opposition at his confirmation hearings last spring, but told senators his record backing workers was strong.

"If we're going to pick and choose cases out of 2,700, I can point you to so many in which I have found for the plaintiff in an employment action, or affirmed the finding of an agency of some sort -- for the worker," he told Democratic Sen. Richard Durbin, who is from Janus’ home state and supports the unions in this case.

While Gorsuch seeks to keep court watchers guessing, Trump's Justice Department has been clear on its position – announcing in December it was reversing course from the previous administration and supporting Janus.

"The [Obama-led] government's previous briefs gave insufficient weight to the First Amendment interest of public employees in declining to fund speech on contested matters of public policy," said U.S. Solicitor General Noel Francisco, in a note to the high court.

Janus, 65, says he does not want to destroy the unions and thinks workers have a right to organize. But he opposes having to pay for a union's lobbying efforts at a time when Illinois is facing a crippling financial crisis.

He is being represented by the Chicago-based Liberty Justice Center.

"In many states, workers are forced to give money to a union whether they want to or not. And when they do that they're funding union politics," said Jacob Huebert, the group's director of litigation. "Not all workers want to support that union agenda, just because they've taken a government job."

Labor leaders oppose so-called "free riding" by workers like Janus, and say they have a legal duty to advocate for all employees:

"Everybody deserves the power to win better wages and benefits and retirement security whether you're in a union or not in a union. That's how we build an economy that works for everyone," said Richard Trumka, president of the AFL-CIO.

About 28 states have so-called "right-to-work laws" that prohibit or limit union security agreements between companies and workers' unions.

States that do allow "fair share" fees say they go to a variety of activities that benefit all workers, whether are in the union or not. That includes collective bargaining for wage and benefit increases, grievance procedures, and workplace safety.

Employees who do not join a union also do not have to pay for a union's "political" activities, but both sides of the issue are at odds over when that would occur.

Court watchers say the legal and political stakes in the Janus case could well determine the future of the union movement.

"I think people who are in public sector unions are very concerned about their viability going forward. Certainly opponents of unions see this case as something that they hope will substantially diminish the power of labor," said Elizabeth Wydra, president of The Constitutional Accountability Center. "But make no mistake, this case is a very serious potential blow to the union movement."

As for Janus, he downplays his role as a potential constitutional gamechanger.

"I just look at it as an average guy just standing up for his own rights of free speech," he said. "I'd kind of like my money instead of going to the union and their causes go toward more civic health such as Boy Scouts, Girl Scouts. There are so many causes that need help and assistance."

The case is Janus v. AFSCME and Madigan (16-1466). A ruling is expected by late June.