U.S. Supreme Court building in Washington, D.C.

The U.S. Supreme Court, deadlocked last term on the constitutionality of mandatory “fair share” union fees that are paid by millions of public-sector workers, agreed on Thursday to step into this arena for a second time in a case that raises alarms among some advocates who fear the court could deal a blow to organized labor.

The high court's decision to review the case Janus v. American Federation of State, County and Municipal Employees adds a second major labor challenge to the new term's docket. On Oct. 2, the justices will hear arguments in a trio of cases in which employers seek to enforce mandatory arbitration clauses prohibiting class and collective actions in employment contracts.

The stakes for unions in the fee dispute are high. A decision striking down the fees as violating the First Amendment rights of nonunion workers would deliver a huge financial blow to public-sector unions.

Unlike last term when eight justices, following the death of Justice Antonin Scalia, faced the union fees question, the confirmation of Justice Neil Gorsuch gives the high court a full bench and no chance for a deadlock. The labor unions appear to have an uphill battle to preserve the fees they claim are necessary to cover the collective bargaining costs for the nonunion workers they also are required to represent.

Read more: Supreme Court's Next Big Union Fight: 6 Key Questions

During last term's arguments in Friedrichs v. California Teachers Association, the unions appeared headed for defeat, with Scalia and the other conservative justices likely votes against them. Gorsuch, who filled Scalia's seat, has not faced the issue. He may cast the deciding vote.

The Janus case was brought to the high court by the National Right to Work Foundation, which has been systematically challenging the fees in litigation around the country. The foundation reports that it has additional cases pending in Pennsylvania, Kentucky, Massachusetts, California, New York and Connecticut.

Mark Janus, a child support specialist for the Illinois Department of Healthcare and Family Services, is challenging the fees paid to the American Federation of State, County and Municipal Employees, Council 3. Represented by William Messenger, he argues the fees violate the First Amendment, and he asks the high court to overrule the landmark union-fee case, Abood v. Detroit Board of Education, which was decided in 1977 and provides the constitutional underpinnings for the union fees. The high court has reaffirmed Abood five times in the last 40 years.

The union, represented by David Frederick of Kellogg, Hansen, Todd, Figel & Frederick, argues that Abood was correctly decided. Although the union fees implicate the First Amendment, the union says, they are justified by the interest states have in the promotion of collective bargaining and in the avoidance of so-called free rider incentives, where nonunion employees enjoy the benefits of the union without paying dues.

Illinois Attorney General Lisa Madigan is also a respondent in the case. State Solicitor General David Franklin defends Abood, saying the decision “took into consideration both the State's interest as an employer in bargaining with an exclusive representative and the associational freedoms of public employees to self-organize and to refrain from supporting political speech with which they disagree.”

In two recent decisions, Justice Samuel Alito Jr. signaled to unions and their opponents that Abood as a precedent was vulnerable in the Supreme Court. Alito has said he believed the fees violated the First Amendment and that Abood should be overruled.

There are nearly 11 million union-represented employees in 22 states that don't have laws prohibiting agency fees. Roughly half of those employees—7.8 million—are in the public sector, according to the U.S. Labor Department's Bureau of Labor Statistics.

Mark Mix, president of National Right to Work Legal Defense Foundation, applauded the Supreme Court's decision to take the union-fee dispute.

“With the Supreme Court agreeing to hear the Janus case, we are now one step closer to freeing over 5 million public sector teachers, police officers, firefighters, and other employees from the injustice of being forced to subsidize a union as a condition of working for their own government,” Mix said in a statement.

Lee Saunders, president of AFSCME, called the Janus case “another example of corporate interests using their power and influence to launch a political attack on working people and rig the rules of the economy in their own favor.” Saunders added: The merits of the case, and 40 years of Supreme Court precedent and sound law, are on our side.”

This article was updated with comment about the Supreme Court's order today.