Delta Air Lines is standing by a campaign to dissuade its flight attendants and baggage handlers from unionizing, saying that a union would interfere with the “direct relationship” between airline management and employees.

The campaign—called “Don’t risk it, Don’t sign it”—was created to prevent employees from joining the International Association of Machinists and Aerospace Workers (IAM).

The controversy comes after a post went viral on Twitter showing two posters designed by the company encouraging employees to spend money on recreational activities like video games and sporting events rather than paying union dues. “A new video game system with the latest hits sounds fun. Put your money toward that instead of paying dues to the union,” one of the posters stated.

The tweet quickly drew criticism from many users of the social network, including senators Bernie Sanders (I-Vermont) and Sherrod Brown (D-Ohio).

“Delta’s CEO made nearly $22 million in 2017 while paying ramp agents as little as $9/hour,” tweeted Sanders May 9. “I say to Delta: Stop trying to undercut workers’ right to form a union and negotiate for better wages.”

One webpage on the campaign website titled “the Dangers of IAM” warns employees that 12 IAM officials had been convicted of stealing over $1 million over the last five years, and alleged that high-ranking IAM officials use members’ dues to pay close relatives’ salaries. Another page titled “Rumors” accused the union of falsely attempting to take credit for recent pay raises at Delta.

A spokesman for Atlanta-based Delta said the company has “shared many communications, which on the whole make clear that deciding whether or not to unionize should not be taken lightly.”

“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,” Delta GM-communications Anthony Black said. “Our employees have the best total compensation in the industry, including the most lucrative profit-sharing program in the world. They want and deserve the facts and we respect our employees’ right to decide if a union is right for them.”

But IAM assistant airline coordinator James Carlson said the recent controversy over the poster “was just the tip of the iceberg of a coordinated union-busting campaign that’s been going on at Delta for years.”

“Delta realizes that when employees unionize, they have power and they get a bigger slice of the pie, and that’s exactly what Delta doesn’t want,” Carlson said. “They’re happy with the existing power relationship where Delta is the master, except for the pilot groups and a small group of flight dispatchers that are unionized. Delta makes the rules and you either live by those rules or you leave—and they like that situation. That’s why they oppose their workers’ unionization efforts so vehemently.”

Ben Goldstein,