Justice Neil Gorsuch is expected to cast the deciding vote Monday in one of the landmark cases of the U.S. Supreme Court’s term, a challenge to the constitutionality of mandatory union fees.

Over five-million government employees are required by law to pay dues to public-sector unions. The Supreme Court has closely divided over the legality of forced dues in the recent past, so Gorsuch, as the newest justice, likely holds the decisive fifth vote in Monday’s case.

Under a 1977 Supreme Court precedent called Abood v. Detroit Board of Education, labor bosses are allowed to collect compulsory dues, or fair-share fees, from public employees since union negotiators represent all government workers for purposes of collective bargaining — even if they are not union members.

Also WATCH: Rebecca Friedrichs explains why she fought her union all the way to the Supreme Court

Abood forbids the union from using fair-share fees for political purposes like electioneering. Critics of the decision, mostly judicial conservatives and libertarians, say this prohibition is meaningless, as most activity associated with collective bargaining is deeply political. Therefore, they claim, the fair-share regime violates the free speech and association rights of dissenting workers, made to subsidize an organization with which they disagree.

Such fees generate tens of millions of dollars in revenue for the unions. Losing that income could deal a crippling blow to organized labor, so union stalwarts say criticism of Abood is little more than a political stalking horse for corporate interests.

Several justices in recent years expressed serious reservations about mandatory dues in a pair of labor cases argued between 2012 and 2014. The Court seemed poised to overturn Abood in Jan. 2016, when it heard a direct First Amendment challenge to the fair-share regime. Justice Antonin Scalia died just one month after the arguments in that case, Friedrichs v. CTA. The eight-member panel deadlocked four to four so a binding ruling was never issued.

Shortly after Gorsuch’s confirmation, the high court agreed to revisit the issue in hopes of reaching a definitive decision. The new case, Janus v. American Federation of State County and Municipal Employees (AFSCME), arose in Illinois. The plaintiff, Mark Janus, is a child-support specialist who pays roughly $600 per year to an AFSCME local, though he is not a member and disagrees with a range of positions the group takes.

If the justices split along ideological lines, as they did in the Friedrichs case, Gorsuch will cast the deciding vote. In his first year on the Court, Gorsuch has proved a reliable conservative. He joined Justices Clarence Thomas and Samuel Alito in criticizing decisions relating to LGBT rights and President Donald Trump’s travel ban, while peppering advocates with questions about the text of statutes and the original meaning of the Constitution during oral arguments.

Still, he wrote very little on this subject as a judge on the 10th U.S. Circuit Court of Appeals, so his views are somewhat unknown.

The Trump administration will argue in support of Janus during the proceedings. The decision is something of a reversal for the Justice Department, which has consistently defended the Abood decision under presidents of both parties.

A decision in the case is expected by June.

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