Just when you thought the American auto industry finally could compete again, a radical Socialist wing of the United Auto Workers is fighting to win back union concessions.

Not only did the rank-and-file recently reject concessions at Ford, it revolted against the UAW’s leadership.

The last few years have been horrible for the UAW and its members. Most UAW-represented auto makers and suppliers filed for bankruptcy, forcing the union to accept major concessions on wages, benefits, holidays, vacations, work rules and pensions.

Worse yet, the union has lost more than 100,000 members over the last two years. Indeed, total active membership soon could drop to about 400,000, calling into question whether the UAW is economically viable.

It’s likely the union’s annual income has fallen below its annual expenses. There were even rumors this summer it was unable to make payroll for staff members for a brief period.

But it’s hard to know for sure. For all its claims about being a democratic union, the UAW is not a very transparent organization. It rarely makes union officers available to much of the media.

There always have been dissidents within the union who felt leadership wasn’t fighting hard enough against their capitalist overlords. But this wing, called Soldiers of Solidarity, has been gaining strength over the last four years, attracting more adherents starting about the time Delphi went into bankruptcy.

Ford recently went to the UAW, asking for the same concessions it granted to General Motors and Chrysler. The union’s leadership negotiated the changes in return for the promise of new products for union-represented plants, plus a signing bonus.

But when it was put to the vote, members soundly rejected the measures, in some cases by more than 90%. The rank-and-file rejected the recommendations of its leadership.

Next year, the UAW will elect a new president to replace Ron Gettelfinger, who is nearing retirement age.

UAW-watchers long have considered Vice President Bob King a shoo-in to win that election. But I’m not so sure. Expect the Soldiers of Solidarity to run a vigorous opposition campaign.

As the economy recovers, GM, Ford and Chrysler will emerge as fabulously profitable companies. By eliminating their debilitating legacy costs, they are poised to report record profits in just a few years.

That is going to embolden the UAW to restore the benefits it gave up, and the dissident wing will be beating the war drums the loudest.

Management doesn’t want labor costs to start escalating again. But the union has proved over and over that well-timed strikes can cripple production and force an auto maker to start bleeding billions in a remarkably short period.

Auto executives quickly realize it’s cheaper to cave in than to fight, and then it’s back to the old, “Here we go again.”

The UAW may be wounded badly, but it still has far more clout than most outsiders realize. If it can remain economically viable, the union will be a formidable factor for a long time to come.

John McElroy is editorial director of Blue Sky Productions and producer of “Autoline” for WTVS-Channel 56, Detroit, and “Autoline Daily,” the online video newscast.