Mark Janus is a government employee. He works for the state of Illinois as a child-welfare specialist in the Department of Healthcare and Family Services. His job is to represent children caught in situations of domestic turmoil — a field he chose because children’s well-being is important to him.

“But just because I care about kids doesn’t mean I also want to support a government union,” Janus wrote last year in the Chicago Tribune. “Unfortunately, I have no choice.”

The employees in Janus’s workplace are represented by the American Federation of State, County, and Municipal Employees (AFSCME), a labor union that, under state law in Illinois and elsewhere, has exclusive authority to engage in collective bargaining over wages, hours, and other terms of employment. Naturally, members of the union pay dues. But even non-members are compelled to pay union levies, known as “agency fees,” to cover the benefits of being represented by the union.

Yet coercing workers to pay for representation they don’t want isn’t a benefit. It’s extortion. And it’s particularly galling when those extorted payments are used to fund political speech and public-policy activism that employees have no wish to underwrite.

Two years ago, Janus filed a lawsuit challenging the Illinois law that forces him to pay fees to the union. The Supreme Court recently announced that it will take up the case this term. Janus v. AFSCME poses a straightforward question: Can public employees be forced to subsidize union speech or risk losing their jobs? If the court rules in Janus’s favor, it will restore to government workers a right most Americans take for granted: the right to decide for themselves which causes and organizations they support.

It should never have come to this. Thomas Jefferson rightly declared long ago that “to compel a man to furnish contributions of money for the propagation of opinions which he disbelieves is sinful and tyrannical.” Under the First Amendment, it should have been out of the question for government to force public employees to turn over part of their wages to a labor union they don’t belong to — or, for that matter, to any other political, ideological, or special-interest organization. But in a 1977 case, Abood v. Detroit Board of Education, the Supreme Court ruled that public-sector workers could be required to pay agency fees in the interest of “labor peace” — as long as their fees were used only for the actual costs of collective-bargaining and representing employees, and not for anything political.

That was a blunder. Unlike the private sector, where labor and management are both constrained by market forces, competition, and the need to remain profitable, unions in the public sector face no such limitations. The government agencies that AFSCME and other unions bargain with can’t go out of business or relocate to another state. Their revenue depends not on customer loyalty and sales but on politicians’ decisions about taxes, spending, and public policy.

Consequently, pretty much everything public-sector unions do is political. In Abood, the court tried to distinguish between core collective-bargaining functions, on which nonmembers’ fees could be spent, and overt political advocacy, which could not be charged to unwilling workers. But that distinction is illusory, as Justice Lewis Powell — who rejected the majority’s reasoning — pointed out at the time:

“The ultimate objective of a union in the public sector, like that of a political party, is to influence public decision-making,” Powell wrote. “The union’s objective is to obtain favorable decisions and to place persons in positions of power who will be receptive to the union’s viewpoint. In these respects, the public-sector union is indistinguishable from the traditional political party.”

If that wasn’t clear in 1977, it soon became an inescapable fact of life.

For public-sector unions, politics became all-important. AFSCME, like the National Education Association, the Service Employees International Union, and others, poured vast resources into honing their political clout. On its website, AFSCME boasts that candidates “all across the country, at every level of government” have learned to “pay attention to AFSCME’s political muscle.” The union is not shy about relying on politics to achieve its goals. “We elect our bosses, so we’ve got to elect politicians who support us and hold those politicians accountable,” AFSCME proclaims. “Our jobs, wages, and working conditions are directly linked to politics.”

Former AFSCME president Jerry Wurf put it in a nutshell: “We’re political as hell,” he told Time magazine.

And the agency fees forcibly extracted from Mark Janus — and from hundreds of thousands of other public-sector employees who have not joined the union — subsidize all that politics. That is an ongoing affront to the First Amendment.

It is long past time the Supreme Court repaired its 1977 mistake. Abood should be overturned, and the court should affirm that Powell was right: In the public sector, collective bargaining amounts to political advocacy. And under the Constitution, nobody can be compelled by government to subsidize political advocacy involuntarily.

If unions are to be tolerated in government workplaces, their support and funding must be wholly unforced. Government workers who choose to join and pay dues to AFSCME or some other union are free to exercise their First Amendment rights of speech and association. Equally free should be those who want nothing to do with the union. Free not to join, and free not to pay.

Jeff Jacoby is a columnist for The Boston Globe.