Although no election has been scheduled yet, the Machinists asked that the board intervene and force Delta to stop any alleged practices that go against the law. The board would now investigate the complaint to see if it has merit.

Along with the complaint, the union said it submitted sworn declarations from baggage handlers and flight attendants that “document a clear pattern of interference with employees’ rights throughout the Delta nationwide network.”

“The IAM’s interference claims are completely baseless. Delta’s unmatched culture puts employees first,” an airline spokesperson said in a statement. “We will continue to lawfully communicate with our employees and defend our federally protected right to educate them on the truth.”

The union claims the Delta campaign amounts to “ an effort to decapitate the union movement and to strike fear among IAM supporters.”

The International Association of Machinists and Aerospace Workers submitted its complaint to the National Mediation Board, the federal agency that referees workplace disputes in the airline industry. It accused Delta of violating labor law by “disciplining and firing union activists,” “destroying union campaign materials” and “coercing employees” to vote against the union.

A union trying to organize baggage handlers and flight attendants at Delta filed a complaint against the airline with federal officials Wednesday, just days after Delta was battered online for putting anti-union posters in its break areas .

The complaint ratchets up a long-running fight that spilled into the twittersphere last week when journalist Eoin Higgins put a photo of a Delta poster meant to dissuade employees from unionizing on social media. The poster said “union dues cost around $700 a year” and suggested workers put their wages elsewhere.

“A new video game with the hits sounds like fun,” the poster advised. “Put your money towards that instead of paying union dues to the union.”

The airline has used similar posters in its “Don’t Risk It Don’t Sign It” campaign run by the D.C.-based firm FTI Consulting, which has previously done work on behalf of the oil and gas industry.

There is nothing illegal about an employer putting up such posters in the workplace. But some of the actions alleged by the Machinists in their complaint, such as retaliation against employees, would be violations of labor law if true.

As Higgins reported in HuffPost earlier this week, the campaign has included a “full-court anti-union press,” with leaflets and advertisements in break areas discouraging workers from supporting the Machinists. “Anti-union videos play by our time clocks, anti-union literature is distributed in our break rooms, managers are designated to push the anti-union agenda, and employees are held captive,” said Dan McCurdy, a Delta employee.

The Machinists complaint claims new employees at Delta have been subjected to “nearly hour-long segments” urging against unionization. The union also says workers have been “singled out” for talks with managers and have been photographed by the company at union rallies, which the Machinists claim is illegal surveillance.

Anti-union campaigns are common with U.S. employers, who often pay high fees to “union avoidance” firms for their work. But they rarely strike a nerve online the way Delta’s has. The publicity brought by the posters lured prominent lawmakers into the fray, with Sen. Bernie Sanders (I-Vt.) blasting the airline. He sent a letter signed by eight other senators, including fellow presidential hopeful Sen. Elizabeth Warren (D-Mass.), accusing the airline of “corporate greed.”

“We urge you to end Delta’s anti-union tactics, make it clear to all of your managers that they should do the same, and allow Delta workers to decide the question of unionization free from fear, intimidation or retaliation,” the letter stated.

The office of Sen. Patty Murray (D-Wash.), the ranking member of the Senate Labor Committee, told HuffPost that Murray staffers met with Delta representatives on Tuesday and recommended they publicly apologize and stay neutral in the union campaign. Murray said in a statement that she was “disappointed” Delta didn’t commit to either of those requests.

The story has been updated with comment from Delta.