The International Association of Machinists and Aerospace Workers (IAM) has filed charges with the US National Mediation Board (NMB) against Delta Air Lines, alleging the Atlanta-based carrier’s anti-union activities violate Delta employees’ right to unionize “free from interference, influence or coercion exercised by the carrier.”

News of the lawsuit follows a series of viral social media posts highlighting anti-union communications from the company that generated outrage from social media users and Democratic lawmakers earlier in May. Delta’s anti-union campaign—called “Don’t risk it, Don’t sign it”—was created to prevent flight attendants and ramp employees from joining the IAM.

“The IAM has provided the NMB with evidence showing Delta has run an unlawful, systematic anti-union campaign that includes intimidation, discipline and terminations of union activists,” IAM general-VP Sito Pantoja said. “Last week, the public was able to see what many behind the walls of Delta have always known; that Delta will go to great lengths to suppress their employees’ collective voices.”

The NMB filing alleges that Delta became more emboldened in its anti-union activities after the Board ruled in its favor during three separate disputes with IAM in 2010. Since revamping its anti-union campaign in 2017, IAM alleges Delta has “intensified its efforts and targeted internal leaders in an effort to strike fear among IAM supporters, disciplining and even terminating union activists.”

The suit also alleges Delta has “conveyed threats and provided misinformation” to employees considering unionizing, including by telling employees they would lose flight benefits and profit-sharing if they join IAM. It also alleges Delta sent anti-union flyers to employees’ homes and told them falsely the IAM would sell their personal contact information.

“In break rooms throughout the country, Delta has forced employees to watch anti-union videos in a continuous loop. Additionally, flight attendants and ramp employees have been bombarded with anti-union flyers which Delta has posted on employee break room and walls and placed on flyer stands which caution employees not to trust the IAM,” reads the suit.

Delta GM-communications Anthony Black told ATW that IAM’s interference claims are “completely baseless,” adding that “Delta’s unmatched culture puts employees first.”

“We respect our employees’ right to choose whether or not union representation is right for them and we believe that same respect should be shown by the IAM, an organization that submitted more than 2,000 cards with forged signatures during their last attempt to organize at Delta, a matter referred to the Department of Justice,” Black said. “We will continue to lawfully communicate with our employees and defend our federally protected right to educate them on the truth.”

On May 15, nine Democratic senators wrote to Delta CEO Ed Bastian urging him to end the company’s “insulting and demeaning anti-union communications.”

“It has become clear that Delta’s management has a high-coordinated and strategic plan to suppress the efforts of its over 40,000 workers from voting to join the IAM,” the senators wrote. “The time has come for Delta to respect its workers’ fundamental right to freedom of association and stop bombarding employees with anti-union propaganda.”

The letter was signed by Democratic Sens. Richard Blumenthal (Connecticut), Edward Markey (Massachusetts), Elizabeth Warren (Massachusetts), Sherrod Brown (Ohio), Jeff Merkley (Oregon), Ron Wyden (Oregon), Bob Casey (Pennsylvania), Bernie Sanders (Vermont) and Tammy Baldwin (Wisconsin).

Ben Goldstein,