On Monday the US Supreme Court will hear Janus v. AFSCME. At issue are the constitutionality of laws in 22 states — including New York — that force public employees who do not want to belong to unions to pay unions “agency fees” for bargaining collectively on their behalf. The plaintiff — Illinois state worker Mark Janus — argues that agency fees violate his First Amendment rights of freedom of speech and association by compelling him to underwrite union political activity with which he disagrees.

If the court sides with Mr. Janus, it would spark the biggest change in government labor relations and state politics in New York since the passage of the Taylor Law in 1967, which first established ground rules for public employee unions.

Now, government unions in the state could lose 20 to 30 percent of their members and money.

Union strength would then reflect the workers who actually want its representation. The balance of power in state politics would shift.

The ability to collect agency fees has inflated the power of New York’s government unions. The unions argue that because all workers benefit from union representation, all workers should bear the costs. Following their logic the law allows public unions to set nonmember agency fees at nearly the same amount as union dues, which creates a strong incentive for most workers to join the union. Today, 67.4 percent of public employees in New York belong to unions, the highest rate in the country.

Membership translates into money. New York’s unions collect over $860 million a year from some 1 million government employees.

Those dollars have made government unions a political powerhouse. An informative Empire Center report shows that from 2013 to 2016, New York government unions spent $43.2 million to lobby state and local governments, which is more than the four largest private-sector interests (hospitals, real estate, tobacco, and trial lawyers) combined.

But public unions don’t just seek to influence already elected officials, they also seek to determine who gets elected. From 2013 to 2016, New York’s government unions spent more than $52 million on state and local elections, according to the Empire Center. Amazingly that sum approached the $63.9 million collectively spent by Gov. Cuomo, the New York State Democratic Committee and state Senate Republicans on their 2014 election campaigns.

Furthermore, the vast majority of public union political activity seeks to aid the left side of the political spectrum, as the unions are closely aligned with the Democratic and Working Families parties.

That’s where the First Amendment problem arises. Workers like Janus feel that they are being compelled to subsidize or affiliate with a political party, or at least a political perspective, as a condition of keeping their job. It is a requirement that is nearly unique in the American labor market. It’s hard to imagine being required to contribute to the Sierra Club or the National Rifle Association in order to hold a job — even though those organizations provide people benefits that they do not pay for.

The unions argue that a remedy for such impingement on First Amendment rights already exists. Workers who dislike union politics just need to withdraw from the union and opt-out of the political spending (usually by writing an annual letter). The union will then reimburse them for the percentage of their agency fees it says it spent on politics.

New York’s government unions could lose up to $110 million a year in agency fee revenue.

But this arrangement stacks the deck in favor of the union. If workers are unsure of their rights or forget to write the opt-out letter, they can subsidize political activity. In addition, the union controls the budget and decides how much it spends on political versus collective bargaining. And in the government context it is it hard to separate political activity from collective bargaining because both are directed at the government.

Even if a line could be drawn between the two, Janus contends that he is still underwriting political activity because collective bargaining in government is inherently political. The subjects up for negotiation — employee pay, benefits, and work rules — are ultimately political decisions about how to allocate tax dollars and how public services should be carried out.

If the Supreme Court accepts these arguments, New York’s government unions could lose up to $110 million a year in agency fee revenue from non-members right off the bat. Further declines in membership and money might ensue. But workers would finally have the choice of where to send their money and with whom to affiliate.

Even without agency fees public unions are likely to remain potent political forces. But the Supreme Court could take them down a peg or two. Such an outcome would have the benefit of restoring some balance to New York’s political landscape.

Dan DiSalvo is a senior fellow at the Manhattan Institute and an associate professor at the City College of New York-CUNY. He is the author of “Government Against Itself: Public Union Power and Its Consequences” (Oxford).