The Supreme Court will hear arguments Monday in a case that could shrink government unions and their campaign war chests by as much as two thirds, with potentially devastating consequences for the Democratic Party in a competitive election cycle.

The case, Janus v. AFSCME, challenges the money that public unions representing teachers, firefighters, nurses and other government employees collect from non-members to cover their share of collective bargaining costs. Democratic candidates and causes rely on these unions for more than $100 million in contributions every election cycle, and for armies of workers dispatched to staff phone banks and canvass neighborhoods across the country.

Plaintiff Mark Janus, an Illinois state worker who declined to join the American Federation of State, County, and Municipal Workers, argues in his lawsuit that the payments he’s compelled to pay the union violate his First Amendment rights.

The facts of the case are near-identical to those in Friedrichs v. California Teachers Association, on which the high court deadlocked in 2016, following the death of Justice Antonin Scalia. The addition of Justice Neil Gorsuch is widely expected to give plaintiffs the fifth vote they need to outlaw the non-member fees.