Make no bones about it. Teachers unions are reeling from a game-changing decision from the U.S. Supreme Court, issued the same day as the news of Justice Anthony Kennedy's retirement, so largely overlooked in news coverage. The public may not have much noticed, but unions feel they are standing at a precipice, not at all certain they can maintain the power they're long accustomed to wielding.

After the high court sided with Janus in Janus vs. AFSCME, public-sector workers will no longer be required to contribute to their unions, something nearly half of all states — including Minnesota — require regardless of whether teachers choose to belong to the union. The nation's largest union, the National Education Association (NEA), having just held its annual convention in Minneapolis, expects to be hard hit. It's anyone's best guess how many of the 78,000 active teachers who currently contribute to the Education Minnesota union will opt out in the years ahead, but the initial hit will almost certainly include some 7,000 teachers who have already registered their discontent over having been forced to contribute.

The fast-flowing pipeline of dollars from teachers to unions ($600 million a year, nationally) is bound to be disrupted, and here's why. Independent surveys consistently report that only half of all teachers see their union as "essential" and that many see dues as too high, political activity as too leftist (only half of all teachers voted for Hillary Clinton) and positions on education issues counter to schools' paramount interests. And with much of this disaffection skewing toward younger teachers, the unions have their work cut out for them.

However, the NEA has a strong foundation on which to build. On its core functions, nearly all teachers give high marks: defending against claims by parents and students, going head to head with school districts, and bargaining for greater pay. But as any PBS station manager can tell you, just because viewers wouldn't have missed an episode of "Downton Abbey," it doesn't mean they donate during Pledge Week.

We should soon expect the launch of a Madison-Avenue-worthy campaign making the case for membership and the sensible notion that teachers have a personal responsibility to pay their fair share. The NEA just announced its intention to fuel more teachers strikes this fall.

There's one move that the NEA and its sister union, the American Federation of Teachers, could make that would keep more teachers in the fold, but it's not clear that current leaders at either union will consider it. They should rethink the combative stand on issues that directly affect teachers and students, starting with their defense of really weak or toxic teachers.

With ample cash on hand, unions have long taken to an extreme their obligation to defend every teacher — which is why some districts estimate the cost of firing a single teacher at more than $200,000. With the two unions being the largest contributor to American political campaigns, state laws they pushed to pass now make it impossible to fire a mathematics teacher who can't teach math. A drunken math teacher? Maybe, but by no means assuredly.

More than half of all teachers don't agree with the unions' to-the-death defense of some teachers. That's because there's nothing that drags teachers down more quickly than having to work alongside teachers who don't do their jobs well. When former teachers cite poor working conditions as the most common reason they leave the profession, they generally aren't referring to unclean bathrooms. They want to be surrounded by teachers who care as much as they do.

Most teachers also recognize the harm yielded by seniority rights, though cherished by unions. That's because they saw firsthand the harm of such rights. As novice teachers, many were assigned to impossible jobs at the toughest schools, as veteran teachers had moved on to take less-challenging positions. Few teachers have fond memories of their first year of teaching, knowing how they unwittingly let their students down.

Well before Janus, teachers unions were struggling with declining membership. The Janus decision is a symptom, not the source, of unions' troubles. It would be a mistake to wish for the demise of teachers unions, as there needs to be a check on school districts' own considerable, often callously inflicted, power over teachers. However, unions cannot continue business as usual, giving higher priority to individual teachers over the health of the teaching profession at large or showing preference to one group of teachers (veterans) over another (novices), particularly when placing students at risk. The path to a more certain future lies in those lessons.

Kate Walsh is president of the National Council on Teacher Quality, a nonprofit, nonpartisan research and policy organization.