Last week, the Michigan Supreme Court affirmed teachers’ right to leave their unions whenever they want. The court decided not to weigh in on a union appeal of a 2015 decision that struck down the Michigan Education Association’s (MEA’s) “August window,” which had helped retain members by requiring that they either leave in August or wait another year to do so. This decision is another blow to the state’s unions. Moreover, its similarity to another case making its way to the U.S. Supreme Court could prove that for teachers’ unions, as goes Michigan, so may go the nation.

This is not the first blow to Michigan teachers’ unions. In 2012, Michigan became a right-to-work state when it passed legislation prohibiting contracts that include agency fees. While all teachers’ union members pay dues, the MEA had been allowed to compel all teachers, including non-members, to pay agency fees — a portion of dues that cover costs of contract representation but not political activities. This was meant to prevent teachers from “free-riding” or getting representation for nothing. Agency fees prop up total union membership because they effectively reduce membership cost. Losing them hit Michigan’s teachers unions hard: Five years later, the MEA had shrunk by 25 percent.

This June, the Supreme Court will rule in Janus v. AFSCME on whether agency fees — still legal in 21 states and D.C. — are constitutional. MEA membership losses are a preview of what could happen nationally if the court rules in favor of Janus. If the pattern established in Michigan after 2012 holds true, national teachers’ union membership could decline between 20 and 25 percent. This is arguably a conservative estimate since, if the August window had not been in place in Michigan in 2012, the MEA’s membership decline would likely have been even more substantial.

While Janus could bring an end to agency fees nationwide, another case — Yohn v. CTA could bring an end to opt-out restrictions like the August window. Waiting in the wings until after Janus is decided, Yohn v. CTA was brought by Ryan Yohn and seven other California teachers who claim that the California Teachers Association’s (CTA) opt-out restrictions are overly burdensome and unconstitutional. “The opt-in/opt-out issue is just as much a First Amendment issue as the compulsory dues issue,” Yohn argues.

With Justice Neil Gorsuch’s appointment reestablishing the Court’s conservative lean, unions are already preparing for a post-Janus world without agency fees. The American Federation of Teachers and National Education Association (NEA) are pushing local affiliates to bolster enrollment through more aggressive recruiting and tactics, such as extended due-paying agreements, that make it harder for members to opt out of unions. However, a ruling in Yohn that lifts opt-out restrictions would also undermine some of these preemptory counterpunches.

Union-adverse rulings in Janus and Yohn could ultimately lead to much greater losses for national unions than already seen in Michigan. Take Wisconsin, which not only eliminated agency fees but also passed 2011 legislation that enacted a bundle of other policies weakening unions. Wisconsin’s NEA membership decline approached 60 percent after five years, dwarfing those in Michigan. If the Supreme Court strikes down both agency fees in Janus and opt-out restrictions in Yohn, membership losses in agency-fee states could look more like Wisconsin’s than Michigan’s.

By ending key supports for union membership, Janus and Yohn could certainly create a strong headwind for teachers’ unions and result in significant membership losses. While teachers’ unions will surely survive, the question is, what will they look like? In anticipation of losing agency fees, Mary Kay Henry, president of the Service Employees International Union, has reportedly been “preparing [her union] to become a voluntary organization.” Teachers’ unions will likely have to do the same, which could require reprioritizing their activities, perhaps by reducing national political advocacy and redoubling their efforts to serve members at the local level. It’s impossible to predict exact outcomes, but if they follow Michigan’s path, America’s teachers’ unions will look markedly different tomorrow than they do today.

Nat Malkus is a resident scholar and deputy director of education policy studies at the American Enterprise Institute. Brendan Bell is a research assistant at AEI.