The nonprofit National Right To Work Legal Defense Foundation Committee submitted a brief Tuesday asking the Supreme Court to weigh on the case of Janus v. AFSCME and rule that mandatory union fees for public-sector workers are illegal.

Janus is one of several high-profile cases vying for the Supreme Court's attention on the issue of mandatory union fees, which could amount to a serious blow to labor unions should the fees be outlawed.

"For too long, millions of workers across the nation have been forced to pay dues and fees into union coffers as a condition of working for their own government. Requiring public servants to subsidize union officials' speech is incompatible with the First Amendment," said Mark Mix, president of the foundation, which is representing the plaintiff, Illinois Department of Healthcare and Family Services worker Mark Janus.

The case asks whether Janus can be forced to pay a "security fee" to the union as a condition of employment. Such fees are common in public-sector union contracts. Janus argues the fee violates his First Amendment rights because he doesn't support the union and doesn't want to subsidize its activities.

Unions argue that they are owed the fees to compensate for their collective bargaining on behalf of the workers. The Supreme Court ruled that such fees were legal in a 1977 case called Abood v. Detroit Board of Education. However, under Chief Justice John Roberts the court has indicated that it is willing to revisit the issue.

Last year, the justices appeared to be on the verge of narrowly overturning Abood in a case called Friedrichs v. California Teachers Association. Justice Antonin Scalia's death shortly after oral arguments in the case is widely believed by court watchers to have prevented a fifth vote to overturn the Abood precedent, leaving the question unresolved by the court. Scalia has since been replaced by Justice Neil Gorsuch, another conservative.

In the brief, the right-to-work foundation argues that Abood conflicts with the court's decision in the 2014 case Harris v. Quinn, which ruled that state-subsidized Illinois homecare workers were not eligible to be unionized. "Any case that followed Abood in failing to apply strict or exacting scrutiny to a compulsory fee will, just like Abood, conflict with cases in which the court applied that scrutiny to an instance of compelled speech or association," it argued.

Another case in appeals court, Yohn v. California Teachers Association, which makes the same argument against the union fees, also may be taken up the Supreme Court this year. A third case, Bain v. CTA, argues that fees charged by unions to cover political expenses unrelated to collective bargaining are unconstitutional under the First Amendment and dissenting public-sector workers shouldn't have to pay them.