Why in the world is the NJEA opposing union man Sweeney?

Labor unions — public-employee unions, in particular — are about as popular with New Jersey Republicans as Colin Kaepernick is with Donald Trump.

Which is why the New Jersey Education Association’s expensive war against Democratic state Senate President Steve Sweeney is so bizarre. The union loses even if it ousts Sweeney (D-West Deptford) and replaces him with Republican Fran Grenier, the pro-Donald Trump, pro-Chris Christie Republican whom the union somehow endorsed in District 3.

New Jersey Democrats — especially Sweeney, an official with the ironworkers union — have historically been very supportive of organized labor in general and public-employee unions in particular. That’s why the NJEA has endorsed Democratic gubernatorial candidate Phil Murphy (who is trying to maintain an uncomfortable silence on the whole Sweeney/NJEA flap).

Even if Grenier improbably beats Sweeney, the next Senate president, who controls what bills make it to the floor for a vote, would likely be another Democrat. And the Dems will be furious about the NJEA’s successful power play. They certainly won’t be in a mood to advance the union’s agenda. And Republicans certainly aren’t going to do it. So, what will the NJEA have won?

Rank-and-file teachers certainly can’t be too happy about seeing their union dues used to elect a man unlikely to support union values.

Nor will this crusade win the union any points from taxpayers (read “parents of schoolchildren”), who will easily recognize it for what it is — a classic, bare-knuckle Joisey power play by people who are supposed to be all about the classroom, not the political playground.

The dispute between Sweeney and the NJEA goes back to 2011, when Sweeney gave crucial bipartisan support to Christie’s public-employee pension and health-benefit reforms. Then, when the governor refused to make the pension payments called for in those reforms, the union called for a constitutional amendment requiring the full pension payments. Sweeney initially promised to support such a measure, but then refused to post a bill calling for a referendum on the proposed amendment. Then, he accused the union of extortion, saying it threatened to revoke its support of Democratic candidates as revenge.

And, so, here we are. The NJEA and a super PAC supporting its agenda are expected to spend $6 million in the race. The Democrats will be forced to spend at least that much in Sweeney’s district — and every dollar spent in that presumably safe district will be unavailable to spend in more competitive districts, potentially hurting Democrats statewide.

There’s only one reason for all this. The NJEA, the state’s largest public union and one of the largest political spenders in the state, is sending a Tony Soprano-like message to the Legislature:

Do. Not. Ever. Dare. To. Mess. With. Us.

Never mind that working to stymie Democrats and elect Republicans won’t help the NJEA advance its issues.

Or help actual teachers. Or students.

Oh, yes — the students. Remember them? Education is supposed to be about the students. The teachers union is supposed to be about helping teachers do a good job teaching those students.

But this expensive, undoubtedly futile political fight simply cements the NJEA’s reputation as just one more bully on the playground.