Police and Missouri National Guardsmen stand guard as protesters gather in front of the Ferguson Police Department, in Ferguson, Mo., in November 2014. (Jeff Roberson/Associated Press)

Supreme Court cases have a way of changing American society far beyond the intentions and expectations of the litigants who start them — or even the justices who decide them.

Marbury v. Madison began as a fight over federal appointments and ended establishing the court as the final arbiter of constitutional issues.

In 1967, Loving v. Virginia struck down all state laws against interracial marriage. Today, it is being cited as precedent for a much broader right to marry that encompasses same-sex couples.

So anyone who cares about the issues of law enforcement and race that have been raised so dramatically in Ferguson, Mo., North Charleston, S.C., Staten Island and Baltimore should care about Friedrichs v. California Teachers Association, which the justices might hear as soon as next fall.

What could a case about teachers have to do with cops? A lot. Rebecca Friedrichs and several colleagues object to their state’s prevailing system of mandatory dues, under which they must contribute to the California Teachers Association, whether they agree with its positions in collective bargaining or not.

In 1977, the Supreme Court upheld mandatory dues in the public sector as a permissible means of avoiding “free-riding” by union-represented employees. The court feared that would spawn destabilizing conflicts among workers — as it had in unionized private industries.

Later, the justices held that employees may withhold the part of dues that goes to union lobbying and political activity, but the exercise of that right has proved difficult in practice, and public-sector unions have accumulated vast political funds and vast political power.

What’s potentially revolutionary about Friedrichs’s case, therefore, is her request that the court bar any mandatory dues in the public sector — even those that ostensibly fund only collective bargaining.

Her clever, and convincing, argument is that, unlike labor negotiations in the private sector, bargaining in the public sector inherently touches on policy issues: Wages, benefits and work rules unavoidably affect taxes, spending and governmental efficiency.

In this context, mandatory dues amount to “compelled subsidization” of a union’s “public advocacy,” as Friedrichs’s petition to the Supreme Court puts it, which a partial opt-out cannot remedy.

And that violates Friedrichs’s First Amendment right to free association and expression, in that it makes her underwrite the propagation of policy views she does not necessarily support.

Now you see why this could be a blockbuster. A victory for Friedrichs would stop the automatic flow of members’ money to public-sector unions in 26 mostly blue states whose laws currently allow it, including not only California but also such public-union strongholds as New York and Illinois.

And that could very well include unions representing the police.

Among the factors that contribute to police impunity, where it exists, the political power of police unions belongs near the top of the list. Police unions have lobbied fiercely against civilian review boards and other reforms aimed at curbing alleged brutality; they demand elaborate procedural protections for officers accused of abuse.

Remember the post-Ferguson campaign to reduce “police militarization”? Police unions opposed it. As for “mass incarceration,” among the staunchest supporters of draconian sentencing laws have been correctional officers’ unions, for whom full prisons mean full employment.

Republicans and Democrats compete for endorsements from well-funded law-enforcement unions. Notably, Wisconsin’s governor, Scott Walker (R), and Republican-controlled legislature exempted police unions, many of which supported Walker’s election campaign, from the public-sector collective-bargaining limits the state enacted in 2011.

Meanwhile, many Democrats advocate criminal-justice reforms. But, since their party depends on teachers unions and other civilian public-sector unions, they support mandatory dues payments — which fund police unions’ resistance to criminal-justice reform.

To be sure, a ruling for Friedrichs would sweep away not only more than two dozen state laws but also the court’s own 1977 ruling in favor of mandatory dues in the public sector. That controlling precedent is still on the books, which explains why the lower courts made short work of her suit — and, indeed, why her lawyers basically consented to those defeats.

They wanted to tee up their petition to the justices, who have issued two recent rulings that raised questions about the continued applicability of the 1977 decision but didn’t quite overrule it.

Friedrichs’s lawyers hope that this means the court is, indeed, ready to declare that precedent outmoded and replace it with a constitutional ban on mandatory union dues in the public sector.

Would that be judicial activism? Perhaps, but activism is no dirty word for this court; witness its willingness to revamp campaign finance laws or take on challenges to the “one man, one woman” marriage laws of most U.S. states.

If the justices agree to hear Friedrichs’s case, it would strongly imply that they want to transform labor law, too — and, with it, the country.

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