Last year, school board races dominated the Colorado political universe. Usually sleepy affairs drawing ballots from only the most steadfast voters, the off-off-year election in 2015 drew national attention thanks to the recall of three school board members in Jefferson County. Throw in the biennial battles in education hotbeds such as Douglas County and the secretary of state’s election updates became must-see-television on election night.

By the next day, it became clear that teachers unions had a very good night. Two years before, the three-member reform majority swept into Jeffco while their neighbors to the south defended every seat in Douglas County. This year, teachers unions gathered signatures to recall members of the Jeffco board in record time. They re-doubled efforts and, not only won all five Jefferson County school board seats, but also all three Douglas County seats bringing that board to a razor-thin 4-to-3 precipice.

Of course, running up to the election, the unions bristled at any suggestion that the campaigns were anything but a pure “grassroots” movement. It was “for the kids!”

In December, after mandatory Department of Labor filings and local campaign finance complaints revealed huge sums of cash flowing from unions to the campaigns, the war had already been won. When it became public that the “grassroots” were as real the AstroTurf in New Orleans’ Superdome — the National Education Association, Colorado Education Association, and Jefferson County Education Association gave nearly $300,000 to the pro-recall JeffCo United organization, or 99.9% of its total contributions — the new board members had already been seated.

That’s just politics. The teachers unions played their hand, and they played it beautifully.

But the United States Supreme Court may be about to rain cold water on their parade. As the calendar turned over to 2016, the Supreme Court heard oral argument in Friedrichs v. California Teachers Association. At stake are the member dues that allow the unions to make the massive political contributions like those made in Colorado. An adverse ruling could cripple teachers unions.

Brought by a California teacher, and supported by education reform advocates, the case centers on mandatory “agency fees.” In many states, teachers must pay these fees to unions to cover the cost of collective bargaining benefiting all teachers. In theory, because every teacher gains the benefit of collective bargaining, each should be required to contribute their “fair-share.” At the same time, teachers can usually request to “opt out” of dues paid to support union political causes.

For example, a teacher paying $1,000 per year in dues may be paying $650 per year in agency fees and $350 in political support. While the teacher may opt out of the $350, the annual $650 is mandatory. The Friedrichs case could make the full $1,000 optional.

If the Supreme Court rules in favor of Friedrich, teachers unions fear a substantial exodus of members and funds. Individual teachers could still reap the benefit of collective bargaining while letting others pay for it and pocketing $650 for themselves — a pretty hefty portion of many teachers’ salary. According to union advocates, that is the “free-rider” effect agency fees prevent. A single teacher stopping dues payments won’t make a substantial difference, but the aggregate of thousands could bleed the union dry.

In Colorado, the case will not have the same dramatic individual effect as in California. While not a pure right-to-work state, Colorado generally allows much more freedom for employees to choose whether to pay dues or not. But money is fungible. Should the court rule in Friedrich’s favor, the $300,000 spent on recalls may be needed elsewhere in 2017 and end up cutting the teachers unions’ current party short.

Mario Nicolais, an attorney and legal scholar, is general counsel at Vivage Quality Health Partners.