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A long-standing federal criminal investigation is roiling a powerful labor union.

The current United Auto Workers’ contract with the “Big Three” automakers expires Saturday, and difficult negotiations are underway, with much speculation about an impending strike.

If that happens, it’ll be at General Motors, this year’s strike target.

The UAW represents nearly 150,000 hourly workers at Ford, GM and Fiat Chrysler, and there’s a lot riding on a settlement that all the parties can accept.

After all, the American auto industry plays a big role in the national economy.

But UAW officials and members have more on their minds than just contract negotiations this year, because there’s another party — an unwelcome one — that has interjected itself between management and labor.

It’s the federal government, whose lawyers and investigators are probing widespread criminal behavior by top UAW and car-company officials.

The government has already obtained multiple convictions involving a conspiracy between officials at Fiat Chrysler and the UAW to sell out employee interests.

Here’s the way that goes — management pays off top union officials, who betray union members by living the high life at their expense.

This investigation has already lasted several years, but it’s nowhere near over.

The FBI recently raided the homes of current UAW President Gary Jones and former UAW President Dennis Williams.

These raids have focused considerable attention on how top UAW officials are using union resources that include members’ dues.

How widely known was it that the union was building a lavish lakefront home for Williams? Further, why is the union operating a money-losing resort and golf club at Black Lake in Onaway, Mich.? That, by the way, is where the union was building Williams’ residence.

Part of the illegal scheme apparently was to use funds that were intended for worker-training programs instead for travel, lodging and meals.

One Fiat Chrysler official, now serving a five-year prison sentence, used part of that money to finance home renovations and buy a Ferrari. At the same time, the official, Alphons Iacobelli, authorized $1.2 million in payments from the training-center fund to a union official and his wife.

It’s unclear where exactly this probe is going. But what’s happened so far indicates that the UAW is a sick institution that has lost its way in terms of its important mission — representing its members.

That was made clear in a sentencing memo prepared by federal prosecutors in Detroit.

The government contends that “there was a culture of corruption in the senior leadership of the United Auto Workers’ union” in which they took money from Fiat Chrysler that was intended to benefit union members and used it “for their own personal benefit, for the benefit of the union itself and for their own lavish entertainment.”

How bad is it? Well, there’s talk of ultimately putting the union under federal oversight and control, as was done years ago to cleanse the mobbed-up Teamsters.

Unions play an indispensable role in evening the playing field when it comes to negotiating wages and benefits between management and labor.

That’s why corruption — either solely involving the union or in a evil partnership between management and top union officials — cannot be tolerated.

Top UAW officials insist nothing is amiss, and it would be no surprise if there are many honest, hardworking people in that union whose priorities cannot be challenged.

But people are already in jail or are on their way. It won’t be long before they’ll have company.

What impact will that have on the current negotiations? No one can say with certainty.

But whether there’s a strike or not, top UAW officials face an inescapable reckoning with prosecutors and their members.