The Supreme Court will wade into a clash between organized labor and conservative groups Monday in a case that could overturn decades-old precedent and deal a potentially crippling blow to public sector unions.

The case is one of the most contentious in a pivotal term, and protesters from both sides are expected to flood the Court Monday morning.

At the center of the debate is a 1977 Supreme Court opinion known as Abood v. Detroit Board of Education that says while non-members of public sector unions cannot be required to pay fees for a union’s political activities, they can be required to pay so-called “fair share” fees pertaining to issues such as employee grievances, physical safety and training.

The Abood decision was a careful compromise when it came down 41 years ago, but in recent years, some conservative members of the Supreme Court have publicly questioned whether it should be overturned.

Today, 22 states have laws on the books that allow broad fair share fees for public employees.

All eyes on Gorsuch

At the arguments, all eyes will be on Justice Neil Gorsuch. Back in 2016, the justices heard a similar challenge and seemed poised to overturn Abood, but then Justice Antonin Scalia died after oral arguments and the court announced a 4-4 split. Now Gorsuch’s vote could be critical.

The case is brought by Mark Janus, an Illinois public sector employee. He says that because he is a government employee, issues germane to collective bargaining are inherently political. He argues that the First Amendment protects him from having to support such political expression.

He is represented by groups such as the National Right to Work Legal Defense Foundation and the Liberty Justice Center who argue that approximately 5 million public sector employees are required — as a condition of their employment — to “subsidize the speech of a third party that they may not support, namely the government-appointed exclusive representative.”

Jacob H. Huebert, one of Janus’ lawyers, says his client specifically does not support the union’s advocacy for increased spending, especially given Illinois’ current fiscal condition.

“A lot of people in the private sector in Illinois are struggling with an increased tax burden and a stagnant economy and Mark Janus doesn’t think it’s fair to put more demands on his neighbors at a time like this, “said Huebert.

Trump officials: First Amendment at stake

The Trump administration is supporting Janus in the case, changing course from the Obama administration in the 2016 case. “The 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,” wrote Solicitor General Noel J. Francisco.

But Lee Saunders, president of the American Federation of State, County and Municipal Employees believes that the case boils down to corporate interests attacking the rights of workers.

“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 to work for a loved one,” he said in a statement.

The unions point out that they are required by law to represent all employees regardless if they are members, and that no one is required to join the union. They say that if non-members don’t have any obligation to pay fair share fees for the collective bargaining obligations, they would become “free riders,” benefitting from the representation without sharing the costs. In addition, the coffers of public sector unions would suffer if non-members were able to get services for free.

They say the 40-year-old Supreme Court decision has left the decision in the hands of the states to decide how to proceed.

Some on the right siding with unions

It’s an argument supported by some Republican state lawmakers who have filed a brief supporting the unions and arguing the decision should be left to the states.

“As things stand, states can determine for themselves which rules work best for their work force and their communities, the court should not use the First Amendment to impose a single rule across the country that harms the ability of public sector workers to bargain together as unions,” said Elizabeth Wydra, President of the Constitutional Accountability Center who represents the lawmakers.

Twenty states and the District of Columbia support the unions, but lawyers for 19 other states have filed briefs on behalf of Janus.

Those states agree with arguments made by business groups who are aiming to strike a blow at the financial structure that supports public sector unions.

Lawyers for the National Federation of Independent Business, for example, argue that the issues unions advocate for are “unfavorable” to small businesses.

“Public employee unions have become powerful in many states, shutting out other voices like the voice of small businesses who cannot afford more costly regulations or higher taxes,” Karen Harned, the group’s executive director said.