The U.S. Supreme Court will hear the opening arguments on Monday in a case that workers and labor leaders fear could deliver a serious blow to public sector unions.

In Janus vs. AFSCME, the court will reconsider its 1977 decision stating that public employees who choose not to join unions can still be required to pay partial union dues, to help fund unions' collective bargaining negotiations, which hold benefits for all workers.

While AFSCME is the union that represents state and municipal workers, supporters fear that a decision in favor of Mark Janus—the individual plaintiff backed by a network of billionaires, corporate interests and right-wing ideologues determined to undermine organized workers—would threaten labor in all sectors across the country. As Andrew Hanna and Caitlin Emma wrote at Politico on Sunday:

The financial blow dealt by a Janus victory would far exceed the loss of nonmember fees, thanks to what economists call a "free rider" problem. Unions are compelled by law to represent all workers within a collective-bargaining unit—not just dues-paying union members. If workers are permitted to quit the union and still benefit from collective bargaining agreements without paying non-member fees, union non-membership will become much more attractive.

Wealthy conservative donors including the Bradley Foundation and the Koch brothers have contributed to the "right to work" cause represented by Janus for decades. The National Right to Work Foundation and the Illinois Policy Institute, which represent the plaintiff, have received hundreds of thousands of dollars in donations from the groups.

#JanusvAFSCME did not grow from an organic, grassroots challenge to union representation — it's part of a broader billionaire-financed agenda to weaken unions and shift power away from ordinary workers. #UnrigtheSystem #ItsAboutFreedom https://t.co/NUMiBTHYLN — Public Citizen (@Public_Citizen) February 26, 2018

The data is clear: When labor unions suffer, wages for ALL workers go down and a greater share of wealth goes to the elite. That's why right-wing millionaires are bankrolling #JanusvAFSCME. https://t.co/WPaYFlrcxH — Rep. Keith Ellison (@keithellison) February 26, 2018

Public sector unions and labor advocates including the National Education Association (NEA), Jobs with Justice, and AFSCME rallied in front of the Supreme Court on Monday, keeping up the momentum started by protests in cities across the country on Saturday's Working People's Day of Action.

Using the rallying call, "Unrig the system," leaders including Sarita Gupta of Jobs with Justice spoke out at the Supreme Court against the corporate power behind efforts to weaken unions.

"This case is a culmination of years of attacks on working people," Gupta told the crowd. "It's an assault by wealthy elites on our freedom to create better lives and sustainable futures for our families. Greedy billionaires, divisive extremists, and the corrupt politicians who do their bidding are rigging the rules in their favor in an attempt to gain more power. And we're not going to let that happen."

A ruling in the Janus case is expected by June.