The conservative majority of the Supreme Court delivered a sweeping and historic blow to the labor movement Wednesday, ruling that public sector workers who are represented by unions cannot be required to pay any union dues.

The 5-4 decision in the case, Janus v. the American Federation of State, County and Municipal Employees Council 31, effectively makes the entire U.S. public sector a “right-to-work” zone. As a result, millions of public employees will have the choice to no longer support unions that must continue to bargain on their behalf.

Janus, as the case is known, was widely seen as the biggest judicial threat to organized labor in years, if not decades. The ruling in favor of Mark Janus, an Illinois state employee, has the potential to squeeze some of the largest and most powerful unions in the country, reducing their clout in the workplace as well as in national and local politics.

The case centered around what are known as “fair share” or “agency” fees. In the United States, a union must represent all the workers in a particular bargaining unit, even those who don’t want a union. While no one can be required to be a full-fledged union member, many states have allowed for contracts stipulating that all workers in the bargaining unit must chip in to cover the costs of bargaining and representation. These agency fees amount to normal union dues minus the portion that the union would devote to politics.

Janus, a child support specialist with the state’s health department, claimed that having to pay agency fees to AFSCME still amounted to “compelled speech,” even if the money wasn’t going directly to political ends. Under his argument, public sector unionism is an inherently political activity, since the salaries and benefits that the unions bargain for impact state budgets and the use of taxpayer dollars. Therefore, the reasoning goes, public workers should not have to pay any union fees if they don’t want to, on First Amendment grounds.

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The original Janus lawsuit was filed by Illinois Gov. Bruce Rauner (R). But a lower court determined that Rauner didn’t have standing in the case because he’s not a union member, so Janus intervened with the help of conservative legal groups. As Janus’ lawyer previously told HuffPost, Janus didn’t want to support AFSCME because the union’s salary and benefit demands for workers like himself “would impose an unreasonable financial burden on the state.”

Even though the Supreme Court had previously upheld the constitutionality of agency fees, in the landmark 1977 case Abood v. Detroit Board of Education, the court’s conservatives upended that precedent with the Janus decision.

Allowing workers to opt out of agency fees leads to what unions call the “free-riding” problem. A worker can choose not to pay them but still reap the benefits of the union’s bargaining and grievance process. A downward spiral can ensue: As more workers decide not to support the union, the union becomes less effective, giving more workers reason to bow out.

“It’s a collective action problem,” Martin Malin, a labor law expert at the Chicago-Kent College of Law, explained to HuffPost earlier this year. “It’s a rational decision ― even for someone who wants to be represented in collective bargaining ― to not become a member, because their dues won’t make any [noticeable] difference, and the benefits of collective bargaining are collective goods.”

This collective action problem already exists in so-called right-to-work states, which bar contracts between employers and unions that require everyone in the unit to pay fees to the union. Twenty-eight states and counting currently have such laws on the books; the Janus ruling will affect the estimated 5 million public sector jobs in the other 22 states and the District of Columbia.

The same conservative groups that have been pushing right-to-work laws around the country have been laying the groundwork for a Janus ruling for years. Their wish nearly came true two years ago, when the Supreme Court considered a similar case involving a group of public school teachers in California.

The 2016 death of Justice Antonin Scalia led to a deadlock on the court, buying unions more time. But the confirmation of President Donald Trump’s Supreme Court nominee, Neil Gorsuch, led to the conservative majority needed to assure a ruling against unions. (President Barack Obama’s liberal nominee, Merrick Garland, had been stonewalled by Senate Republicans for 10 months before Trump’s inauguration.)

Supreme Court rules in favor of non-union workers who are now, as an example, able to support a candidate of his or her choice without having those who control the Union deciding for them. Big loss for the coffers of the Democrats! — Donald J. Trump (@realDonaldTrump) June 27, 2018

The likelihood of an adverse ruling has already forced many unions to change the way they operate. The largest unions in the public sector ― the National Education Association, the American Federation of Teachers, the Service Employees International Union and AFSCME ― have begun focusing on “internal organizing,” reaching out to workers they represent in order to get them more involved in the union.

There’s a consensus within labor that if public employee unions hope to survive after Janus, they will have to better engage their members and constantly prove the value of being in a union. Unions that do a poor job of representing workers will likely see their numbers decline, perhaps significantly.

The political ramifications of the ruling will be long-lasting. Unions are a pillar of the Democratic Party, steering money to labor-friendly candidates and helping to run Democratic ground games during campaigns. Janus may force some unions to cut back on political spending as they steer resources toward maintaining their membership levels.

If that’s the case, Republican candidates stand to benefit. As a recent paper from the National Bureau of Economic Research showed, right-to-work laws tend to drive down votes for Democratic candidates for office. Comparing presidential election results in adjacent counties across state lines, the bureau’s researchers found that the Democrat’s share of the vote fell by 3.5 percentage points after a right-to-work law went into effect.

Janus v. AFSCME by Anonymous FBZLGr on Scribd

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This article originally appeared on HuffPost.