WASHINGTON — The Supreme Court is poised to deal a sharp blow to the unions that represent millions of teachers and other public employees, announcing Thursday it will consider striking down the mandatory fees that support collective bargaining.

The justices will hear the case of Mark Janus, an Illinois state employee who objects to paying fees to the union, which represents 35,000 state workers.

The decision, due by June, could prove a costly setback for public-sector unions in 22 states, where such fees are authorized by law. Labor experts have predicted a significant percentage of employees would stop supporting their union if given a choice. The other 28 states have “right to work” laws that forbid requiring workers to join or support a union.

With smaller numbers, public employee unions would lose some of the political power that has made them major forces in some states, such as California, Illinois and New York.

The nation’s four largest public-sector unions criticized the case, calling it “a blatantly political and well-funded plot to use the highest court in the land to further rig the economic rules against everyday working people.”

The unions have had time to prepare for what’s coming. Early last year, the court’s conservatives were poised to strike down these “fair share” fees in a suit brought by a California schoolteacher. But Justice Antonin Scalia died in February 2016, leaving the court split 4-4 and unable to decide the case of Friedrichs v. the California Teachers Association.

Now, the court has agreed to hear a new case presenting the same issue. This time, Justice Neil Gorsuch can — and most likely will supply the fifth vote for a conservative ruling.

The union fees case presents the question of whether to overturn a 40-year-old ruling. In that case, Abood v. Detroit, the Supreme Court said it was reasonable to require all employees, not just union members, to pay to support the cost of bargaining because all of them benefited. By law, the unions are required to represent all employees, including by handling their grievances.

The case before the high court began two years ago when Illinois Gov. Bruce Rauner filed a suit in federal court contending that union fees paid by state employees were unconstitutional. Illinois Attorney General Lisa Madigan stepped forward to defend the state law. Mark Janus, a child support specialist, and two other state employees asked to join the suit on Rauner’s side.

Janus said he objected to paying $44 a month to the American Federation of State, County and Municipal Employees.

david.savage@latimes.com