The Supreme Court is seen April 20. (J. Scott Applewhite/AP)

(Corrections: A similar case was heard by the court two years ago, not Janus. And Aboud case was issued in 1977, not 1997.)

The U.S. Supreme Court is expected any day now to render its decision in the case of Janus v. AFSCME, and it is expected that the conservative majority on the court will oppose public sector unions (though, of course, it could decide not to).

In February, with Education Secretary Betsy DeVos listening in the courtroom, the justices heard arguments in the case involving an Illinois public sector employee, Mark Janus, who sued the American Federation of State, County and Municipal Employees. He argued that a state law allowing the union to charge and collect fees from nonmembers, including him, violates his and others’ First Amendment rights.

The Janus decision could affect laws in 22 states that allow public sector unions to charge and collect “agency” or “fair share” fees from public employees who aren’t members. The rationale behind those laws: Workers benefit from contracts negotiated by unions — whether they’re members or not — and should pay something to support the unions.

A 1977 Supreme Court decision, Abood v. Detroit Board of Education, said that public sector unions could charge and collect fees from nonunion members to help defray the cost of collective bargaining and other activities. Nonunion members, however, do not have to pay for a union’s political activities and can get a refund for that portion of their fee.

Janus argued that he was being forced to pay for political activities that he opposed and that nonunion members paying “agency” fees should not have to sign a letter every year saying they don’t want to be connected to those activities to get their refund.

Public sector unions represent people who work at all levels of government — including teachers, firefighters and police officers. Those that represent federal employees can’t collect agency fees. The largest union of any kind in the country is the National Education Association, a teachers union.

The Supreme Court first heard a similar case to Janus case two years ago and the vote was split 4 to 4 (when there were only eight members on the court). If each of those justices sticks with his or her position, as expected, the deciding vote will come from the newest member, Neil M. Gorsuch, a conservative appointed by President Trump.

There have been different predictions about how teachers’ unions would be affected if the court rules in favor of Janus. Some have argued that the membership of the two major teachers unions, the National Education Association and the American Federation of Teachers, could sharply decline. Others have said an assault on public sector unions could spark a new union movement that could swell membership, but union officials are already planning for the former.

The Economic Policy Institute, a Washington-based nonprofit, has published an explanation of the case and why it matters, saying: “If the court rules for Janus, it will likely have the most significant impact on workers’ freedom to organize and bargain collectively in 70 years.”

Here is that piece, published on the website of the institute, which gave me permission to publish it.