A federal judge said Rauner didn't have the legal right to challenge the IL law, leaving the employees to press ahead with the case.
Justices granted plaintiffs' request for a hearing in Janus v. AFSCME, Council 31-a challenge of an IL law requiring public sector employees to pay agency fees that fund collective bargaining.
The unions say losing those fees would be a heavy blow because there is no incentive for workers to pay for collective-bargaining representation they could receive free. He was one of three employees who took over a lawsuit originally filed by Illinois' Republican governor, Bruce Rauner. That ruling said that states could allow public-employee unions to collect fees from nonmembers to cover the costs of workplace negotiations but not to cover the union's political activities.
In that case, Terry Pell, president of the Center for Individual Rights, a public-interest legal team that brought the case on behalf of the educators in California who objected to mandatory funding of union activities they described as political, said the result was expected.
"People shouldn't be forced to surrender their First Amendment right to decide for themselves what organizations they support just because they decide to work for the state, their local government or a public school", he added.
After the justices deadlocked 4-4 in a similar case a year ago, the high court will consider a free-speech challenge from workers who object to paying money to unions they don't support.
The Illinois case involves Mark Janus, a state employee who says Illinois law violates his free speech rights by requiring him to pay fees subsidizing a union he doesn't support, the American Federation of State, County and Municipal Employees.
Rauner hailed the court's decision to hear the case.
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Twenty-two states have not banned agency fee requirements, covering about 5 million public-sector workers.
But David Frederick, a Washington lawyer representing Janus's union, the American Federation of State, County and Municipal Employees, said union dues are justified by the expense of contract negotiations.
The decision to hear the case sparked a new round of concern among union leaders, who have been fighting for years in courthouses and legislatures to preserve agency fees.
The Pittsburgh Post-Gazette reports that the state Supreme Court's 5-2 ruling reversed a 2015 decision by a lower court that rejected the challenge to how the state allocates funds to schools.
The court will hear arguments early next year and rule by June.
The case is Janus v. AFSCME Council 31.
That case, Friedrichs v. California Teacher's Association, was over the approximately $650 each teacher in California has to pay the union, despite the fact the union's political statements and bargaining activities may be in direct violation of any individual teacher's speech rights.