Today, the US Supreme Court will hear arguments on the legality of how teachers unions are funded in Friedrichs vs. California Teachers Association. The Friedrichs ruling could have monumental ramifications for the future of teachers unions. It is, in the words of United Federation of Teachers President Michael Mulgrew, “the gravest threat to [their] existence since the creation of our modern system of labor law and collective bargaining in 1935.”

Friedrichs differs from high profile judicial defeats unions have faced recently.  Most notably among these is the Vergara ruling in California, which struck down a number of controversial teacher protections unions worked hard to keep, including tenure­ and seniority privileges. The Vergara ruling struck at teacher protections unions want, but not at unions’ structure or influence.

What is it then about Friedrichs that Mulgrew and other union leaders fear? Friedrichs targets the “agency-fee” requirements for non-union members, which for decades have been a lynchpin to the structure maintaining unions’ outsized power.

So what are “agency-fees? I personally ran into them in my first year of teaching public school.  The county HR rep gave me the choice to join the union —which would cost about $40 per month, and provide me with a union vote, representation and liability insurance— or to not join and only pay agency-fees of about $27 a month. Why did I still have to pay either way? Because the union had “exclusive representation,” meaning only they negotiated with the district, and I had no choice but to pay.

I felt this was unfair, but put up with it like a tax I figured I could not avoid. Rebecca Friedrichs and her co-petitioners felt the same way, but they sued.

The Friedrichs case alleges this system of mandatory agency fees violates First Amendment rights of free speech and free association, because non-members are forced to support the union, even if they are not forced to become a member. Unions engage in substantial political speech including lobbying, endorsing candidates, advocating specific policies and contributing to candidates, but claim that the agency fees are only used for representation and not their political activities. Plaintiffs find those distinctions between representation and political activities as too murky to be credible.

There is no predicting the court, but Friedrichs is a strong case that hinges on First Amendment rights.  The current court has proven unafraid of overturning long precedent to protect the First Amendment (remember Citizens United) and the court may indeed be poised to overturn the precedent on agency fee arrangements set in Abood v. Detroit Board of Education. If the court rule against agency fees, it will be a serious blow to unions in half of states that currently have fees in place.

The initial blow would obviously be a substantial reduction of unions’ income streams. However, the short run hit to revenues pales in comparison to the havoc that would result to union membership. For example, in 2011, after Act 10 abolished agency fees in Wisconsin, union membership fell between 30 and 50%. Similar reductions would be expected in the states that currently require such fees.

Removing agency fees totally changes the cost of membership for union members. For instance, Ms. Friedrichs and other California teachers face a choice between paying $650 for representation, or about $1,000 for full union membership.  That $350 cost differential might be worth having a vote in the union, representation in complaints to leadership, and liability insurance. Especially when you are already paying $650, the $1,000 alternative does not seem so high. Remove the required $650 agency-fee, and the real cost to be in the union becomes $1,000 annually, nearly three times the current marginal cost.

Initial drops in membership aren’t the whole story either, as membership should continue to slide over time. This initial shift in the marginal cost of union membership will certainly encourage members who aren’t strongly tied to the union to drop out. Moving forward, unions will face a much harder time recruiting new employees, as union membership becomes a hard sell instead of a default choice. Further, as membership declines, so will the stigma of not belonging to the union, and eventually unions in some places could face the unthinkable: a legitimate challenge to their dominance as “exclusive representatives” in contract negotiations.

Unions won’t live or die by the Friedrichs case; in fact, a defeat could make them more focused, leaner and more effective for their core constituents (an effect the lead plaintiff might celebrate). But make no mistake, Mulgrew is correct that this ruling could undermine the structural supports that have maintained unions as the most powerful player in education politics.