Unionization efforts by Delta Air Lines ramp workers and flight attendants received fresh fuel Wednesday when several U.S. senators, including Minnesota’s Tina Smith, scolded the company’s chief executive for what they characterize as an anti-union campaign of misinformation.

And the International Association of Machinists and Aerospace Workers (IAM), the union with which a contingent of workers are seeking representation, filed an election-interference complaint against Delta with the National Mediation Board.

The union is leveraging the public outcry stirred last week by viral social media photos of signs with anti-union statements posted by management in Delta workers’ break rooms across the country.

“Workers deserve the opportunity to decide for themselves whether to form a union in an environment free of intimidation or misinformation,” Smith, a Democrat, wrote to Ed Bastian, Delta’s chief executive.

“Unfortunately, your current campaign appears to be rooted in anti-union scare tactics rather than in a fair and impartial presentation of relevant information to allow workers to make an informed choice,” Smith wrote.

Last week, photos of posters hanging in or near employee break rooms surfaced reminding workers that union dues cost around $700 a year and encouraging them to instead put that money toward life’s pleasures, such as a video game console, baseball tickets or beer.

The company’s messages were roundly criticized as condescending and misleading. Delta said it was only trying to help its workers make an informed decision about the implications of joining a union.

“The direct relationship we have with our employees is at the very core of our strong culture and it has enabled continuous investments in Delta people,” the company said. “They want and deserve the facts and we respect our employees’ right to decide if a union is right for them.”

In its complaint, IAM suggested Delta’s tactics are illegal and that workers should be able to make a decision free from “interference, influence and coercion.” The union also alleged Delta is surveilling organizers, requiring workers to attend meetings where anti-union messages are shared and retaliating against pro-union workers through firings and disciplinary actions.

Delta responded late Wednesday by calling the interference claims “completely baseless.”

“We will continue to lawfully communicate with our employees and defend our federally protected right to educate them on the truth,” a Delta statement said. The airline went on to accuse the IAM of submitting more than 2,000 cards with forged signatures during the last unionization attempt at Delta, which it referred to the U.S. Justice Department.

Minneapolis-St. Paul International Airport, one of Delta’s largest U.S. hubs, is a hotbed for the organizing efforts — a legacy of Northwest Airlines, which was known for its adversarial relationship with its pro-union workforce.

“Minneapolis is a very pro-union city and a lot of our workforce here were longtime employees who also worked at Northwest, so a lot of their pro-union beliefs carried over to Delta,” said Dan McCurdy, a Delta ramp agent at MSP, who worked for 10 years at Northwest before the 2009 merger. “The atmosphere is pretty tense on the ground because the company is pretty forward in their anti-union efforts.”

The posters and materials were created about a year ago, Delta said.

McCurdy thinks management developed its anti-union campaign, called Don’t Risk It. Don’t Sign It., last year because their organizing efforts were gaining support. The company’s campaign name refers to the federal requirement that organizers get 50% of the workers, plus one, to sign support cards, which would then trigger an election.

“If a union election were held right now we would win it hands down,” McCurdy said. “Our problem now is that we are in a work environment that is hostile toward unions so it makes it difficult to get the necessary cards signed to have the election held.”

A key message of the pro-union group relates to Delta’s growing reliance on what are called “ready reserve” workers, or part-time employees.

“They have made it pretty clear that they want to move toward a freelance-type labor rather than career-oriented type employees,” McCurdy said. “They are a business and you can’t blame them for wanting to make these enormous profits, but I want to make sure that the workers that helped them get there are protected.”

Delta disagrees with that characterization.

“All ready-reserve employees have the opportunity to convert to full-time within five years and they’re paid on the same scale as full-time agents,” a Delta spokesperson said.

About 40% of its ramp workers are ready reserve. The company declined multiple requests for whether that percentage has increased in recent years and whether ready-reserve workers are guaranteed a full-time job in the city where they live.