The Supreme Court on Thursday agreed to hear a new challenge to public-sector union fees arriving from Illinois.

In Janus v. American Federation of State, County, and Municipal Employees, the Supreme Court will look to determine the constitutionality of a 1977 high court ruling — Abood v. Detroit Board of Education — that said public-sector employees who do not belong to a union can still be forced to pay a fee that covers the union's costs in negotiating the contract that applies to all employees.

The Supreme Court looked poised to overturn the Abood decision in Friedrichs v. California Teachers Association in 2016, but Justice Antonin Scalia's death yielded a 4-4 split from the eight remaining justices. Since Justice Neil Gorsuch replaced Scalia, the nine justices' decision to take the case suggests they could be looking to reverse the earlier precedent yet again.

The court's announcement Thursday morning that it was taking the case is potentially momentous for public-sector unions and their members.

The case could rewrite federal labor law, severely limiting unions' ability to automatically demand dues from those members. That would be a serious financial blow as many unions depend on those dues from unwilling members.

"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," said Mark Mix, president of the National Right to Work Legal Defense Foundation, which is defending the plaintiff in the case.

The case specifically asks whether Mark Janus, an Illinois state employee, can be forced to pay a so-called "security fee" to the union as a condition of employment. Such fees are common provision in public-sector union contracts. Janus argues the fee violates his First Amendment rights because he doesn't support the union and doesn't wish to subsidize its activities.

AFSCME counts on those fees. An internal survey the union did in 2015 found only one-third of its members would voluntarily pay dues no matter what and half of its membership couldn't be counted upon to do that. A minority of 15 percent would be certain to opt-out of paying dues.

Unions argue that they are owed the fees to compensate for their collective bargaining on behalf of the workers.

Janus v. AFSCME is a potential replay of the Friedrichs case that nearly overturned Abood.

"The Janus case is an important step and we are pleased the court is taking it up. We are watching closely and hopeful that Janus will receive a favorable ruling, opening the door ... to help end – once and for all – the unconstitutional practice of compelled support for unions for millions of public sector workers," said Terry Pell, president of the Center for Individual Rights, which represented the plaintiff in Friedrichs.

Overturning Abood would effectively give all public-sector union workers rights similar to those of private-sector workers in right to work states, which prohibit security clause in union-management contracts.