The U.S. Supreme Court will hear arguments Monday in Janus v. AFSCME, a constitutional challenge to the dues-like union fees many state and local government workers must now pay. The outcome could shake the foundations of public-sector union power across the nation—especially in New York.

About 73 percent of New York state and local government employees—roughly 1.2 million in all, including many professionals, mid-level managers and administrators—are covered by union contracts. That’s more than double the national average of 39 percent, and the highest percentage found in any state.

New York government unions received at least $862 million withheld from worker paychecks in 2016, including $112 million in “agency fees” from nearly 200,000 government employees who have chosen not to join unions. Government employers will have to stop withholding fees from these non-members if the court rules for Mark Janus, the Illinois state worker who challenged the status quo on First Amendment grounds.

A similar case, brought by a California public school teacher, was headed towards a ruling for the plaintiff before the untimely death of Justice Antonin Scalia, leaving a 4-4 deadlock on whether to overturn the 1977 decision in Abood v. Detroit Board of Education that upheld the constitutionality of agency fees. Court-watchers think Scalia’s replacement, Neil Gorsuch, is likely to side with Janus and overturn Abood.

Assuming that happens, and agency-fee collection ends, New York public employees who had joined their unions thinking they’d be paying one way or the other may reconsider, and many are likely to opt out. The resulting loss of revenues will reduce the overweening political power unions have long enjoyed in New York politics. Between 2013 and 2016, the unions spent $43 million on lobbying, more than the state’s biggest real estate, hospital, trial lawyer and tobacco interests combined, and spent even more during the period to influence elections.

Recognizing the threat to their political dominance, New York unions and their allies in state elective office are aiming to thwart the impact of the Janus ruling—and squelch workers’ rights in the process.

Since last year, they’ve been promoting a bill that would eliminate the long-standing right of government workers to drop out of unions at any time through a simple written notification to employers. Instead, for the first time ever, unions themselves could set and enforce their own opt-out terms, constructing hoops through which workers would have to jump before escaping from the dues dragnet. NYSUT, for example, is working to limit that union’s opt-out period to a narrow window at the beginning of the school year—the busiest and most distracting time of year for school teachers.

The legislation, originally presented to lawmakers as an innocuous bid to “streamline” and “modernize” collective-bargaining laws, would also would require all government employers to put workers through union pressure-sessions on the clock—making it easier to push these workers into joining while forcing another unfunded mandate on local governments and school districts.

The Supreme Court hearing in Janus will hinge on the plaintiff’s argument that all money spent by government unions—not just dollars spent on lobbying and political contributions—inherently constitutes compelled political speech, and thus violates the rights of workers. In response, the case for preserving agency fees boils down to organized labor’s age-old “free rider” argument: it’s unfair, say the unions, for non-members to benefit from the salaries and benefits determined by collective bargaining.

But the world won’t end for government unions if workers suddenly have a choice. Instead, unions will have to treat their members more like customers and work harder to provide real value for dues dollars—which, in turn, will mean focusing more on member concerns and dialing back the far-flung ideological and political advocacy in which they now often engage. This is hardly unprecedented.

In fact, unions representing federal employees have served their members for decades without raking in compulsory payments—just as New York’s government unions did before they won the right to negotiate for agency fees in 1977.

The Janus case boils down to a simple choice between the vested interests of entrenched unions and the fundamental rights of workers. A win for the workers would ultimately be a win for all New Yorkers.