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Flight-line workers at Boeing Co.'s 787 Dreamliner campus in North Charleston this year voted in favor of having the International Association of Machinists represent them in collective bargaining. It was considered a small but symbolic victory for organized labor in the South. File/Brad Nettles/Staff

A group of nearly two dozen U.S. senators is calling on Boeing Co. to begin negotiations with a labor union representing employees at the aerospace giant's North Charleston campus.

Flight-line workers at the 787 Dreamliner assembly plant voted 104-65 in May to join the International Association of Machinists. Boeing is contesting the election and has refused to negotiate with the IAM until the National Labor Relations Board rules on its appeal.

"Despite opposition, the flight technicians at the South Carolina facility voted to join the IAM earlier this year," according to a letter signed by 23 senators and sent to Boeing CEO Dennis Muilenburg on Wednesday.

"Instead of respecting its workers' choice, Boeing has appealed the election and challenged the legitimacy of the bargaining unit," according to the document. "We oppose these efforts to deny these workers their right to union representation, and we urge Boeing to drop its appeal and immediately begin collective bargaining with its workers through their selected union representative."

Boeing spokesman Victor Scott said "we do not intend to recognize the IAM as the lawful representative of our teammates while the appeal is pending."

"We continue to strongly believe that this micro-unit is prohibited under federal law and we are appealing to the NLRB under the established process," he said.

All of the senators who signed the letter are Democrats, including some of the party's biggest names, such as Bernie Sanders and Elizabeth Warren. South Carolina's senators, Tim Scott and Lindsey Graham, both Republicans, did not sign the letter.

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Boeing CEO Dennis Muilenburg. File/AP

The lawmakers who signed the letter noted that the IAM — which represents 35,000 Boeing workers, mostly on the West Coast — "has been a critical part of Boeing's long, successful history."

"Boeing's accomplishments would not have been possible without the skills, dedication and ingenuity of the IAM members that build your planes," their letter states.

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Robert Martinez, the union's president, thanked the senators in a statement and added: "We hope this effort will encourage Boeing to finally negotiate with its South Carolina flight line and agree on a contract that will make the company and the entire aerospace industry stronger."

The IAM has filed complaints with the federal labor board over Boeing's refusal to negotiate with or provide information to the union. The NLRB has not yet responded to those complaints and has not said whether it will rule on Boeing's appeal.

Union officials appear to be taking a page from Boeing's book in enlisting politicians for help with their cause. In the past, the IAM has complained about Republican politicians using speeches and advertisements to urge workers to vote against organized labor. And last month, Gov. Henry McMaster was joined by governors in Maine, Kentucky and Mississippi in filing a document with the NLRB in support of Boeing's flight-line appeal.

The IAM lost a bid last year to organize production workers at Boeing’s Dreamliner campus and other North Charleston sites. But flight-line workers contacted the union this year with concerns about what they termed unwarranted and arbitrary changes to work rules, threatened layoffs and mandatory overtime on weekends.

Boeing has said the work that flight line employees do is so intertwined with the rest of the facility that the small group can’t be parsed out for collective bargaining. A regional director with the NLRB disagreed in May, setting up the vote.

Boeing is one of the Charleston region’s largest employers, with about 6,800 workers at the Dreamliner campus and other sites in North Charleston.

Reach David Wren at 843-937-5550 or on Twitter at @David_Wren_