Four workers at a Chattanooga, Tenn., Volkswagen plant have filed charges against their employer, alleging it is trying to coerce them into joining a union.

Their filing is the latest turn in a long-running effort by the United Auto Workers to organize the non-union facility.

The workers claim in a complaint filed Wednesday with the National Labor Relations Board that plant management repeatedly told them they would have to join a union if they wanted to see plant production expanded. Otherwise workers would miss out on the opportunities provided by the expansion.

"If VW management was discouraging workers from joining the UAW with threats, there's little question that an NLRB prosecution would have already begun at the UAW's behest," said National Right to Work Foundation President Mark Mix, which is representing the four workers.

The controversy has to do with the German-based Volkswagen's corporate leadership, which includes organized labor. The European nation's law requires that union members get as many as half of the seats on a company "supervisory board."

The boards have considerable influence over the companies, including the ability to hire and fire top executives and oversee production.

A top labor official on the German board, Stephan Wolf, said in June the company will not authorize expanding the Chattanooga plant to include a new SUV line until "it is clear how to proceed with the employees' [union] representatives in the United States."

The German labor officials are using their clout on behalf of their U.S. counterparts. Tennessee is the home of several foreign companies' domestic manufacturing plants. It is also a right-to- work state. Unions have long struggled to gain a foothold there.

In a June interview with Automotive News, United Auto Workers President Bob King said: "If I was a worker, if I was a member of the Chattanooga community, and I wanted to have the best chance of getting new investment and new product, I would want a voice on the world employee council. I would want somebody there representing the interests of Chattanooga."

According to the NLRB complaint, other workers have heard similar messages from company officials. It cites in particular Oct. 6 comments by Bernd Osterloh, vice-chairman of VW and head of VW's global works council, who said the Chattanooga workers must join the UAW before the plant can be expanded.

The complaint alleges that this amounts to a threat against the economic interests of the employees and therefore counts as coercion under U.S. labor law.

Eight workers at the same plant alleged last month in another NLRB complaint that the UAW had engaged in fraud in its effort to organize the facility through a card check election.

The workers said that union officials lied to them, claiming that signing union cards did not count as a vote to join the union when in fact it did. Those workers are also being represented by the National Right To Work Foundation.

Three of those eight workers are also involved in Wednesday's complaint.