Hundreds of union members, including teachers, nurses and construction workers, rallied inside the State Capitol in St. Paul on Saturday to draw attention to a Supreme Court case that could deliver a major blow to organized labor.

The Working People’s Day of Action rally was organized in response to the Janus vs. AFSCME Council 31 case, scheduled to be heard by the high court on Monday.

The justices will consider whether public sector unions may require workers who aren’t members to pay dues to help with collective bargaining.

Union advocates say mandatory dues are justified because nonmembers benefit from their efforts, and that without it, unions would lose influence and revenue. Critics say the requirement is an unreasonable mandate for workers who may object to union stances and how dues money is spent.

The case was brought by Illinois state employee Mark Janus against the American Federation of State County and Municipal Employees.

“The intention of this case at the Supreme Court is to weaken unions by saying that you can benefit from the work a union does … but you don’t actually have to contribute to the work it takes,” said Mary Cathryn Ricker, executive vice president of the American Federation of Teachers and one of the organizers of Saturday’s event.

More than 300 people, many wearing Teamsters jackets and scarves with their locals’ names, heard a dozen speakers endorse collective bargaining and decry efforts to weaken the power of unions.

St. Paul Mayor Melvin Carter opened the rally by saying that unions have been at forefront of historical movements, such as civil and women’s rights.

“When people try to divide us, we know that unions are one of the critical tools for how we say we are united,” he said to cheers and applause.

Angela Byrne, a financial analyst with the state Department of Commerce and a member of the Minnesota Association of Professional Employees, raised a sign reading “United we bargain, divided we beg” from the second level of the Capitol rotunda.

For her, the case is about “trying to get the Supreme Court to make sure that they’re supporting the workers and don’t erode all the rights that we have left.”

Her union ran an aggressive membership drive in the year leading up to the Janus hearing in case the high court rules in his favor.

“It’s [about] being able to have the collective bargaining power to fight for things that we need to live,” she said. “It’s an uphill battle.”

The same question raised by Janus was heard by the justices in Friedrichs vs. California Teachers Association. They seemed poised to rule against the unions when the case was argued in January 2016, but the death of Justice Antonin Scalia the following month resulted in a 4-4 deadlock.

That may have given the unions only a brief reprieve. Scalia’s replacement, Justice Neil Gorsuch, usually aligns with the court’s conservative wing and may well supply a fifth vote against the unions.

The Capitol rotunda rally was part of a nationwide day of action involving dozens of other gatherings. The event was co-sponsored by the American Federation of Teachers, the National Education Association, the Service Employees International Union and AFSCME, the defendant in the case.

Public sector unions are expected to hold a news conference at Christ Lutheran Church in St. Paul following Monday’s oral arguments in Washington, D.C. A high court decision on the case is expected this spring.