Delta Air Lines CEO Ed Bastian on May 22 pushed back against recent criticism from US Sen. Bernie Sanders (I-Vermont) and other Congressional Democrats who have alleged the Atlanta-based carrier has engaged in a targeted campaign to intimidate flight attendants and ramp workers from unionizing.

The controversy ignited earlier in May when a social media posts went viral on Twitter showing posters produced by the airline discouraging workers from joining the International Association of Machinists and Aerospace Workers (IAM). Sen. Sanders penned a letter to Bastian May 15, which was signed by eight other Democratic senators, decrying what he called the airline’s “insulting and demeaning anti-union communications.”

Three more senators joined a separate letter to the company May 17, led by US Sen. Cory Booker (D-New Jersey), in which they said that Delta’s anti-union tactics “are reprehensible and have no place in our society.”

In his letter to Sanders, Bastian agreed the poster that caused controversy on social media “was poorly crafted and not an appropriate communication to our people.” He said it was a year-old poster that was in company break rooms and removed after just one week.

“That’s not who we are, and we have taken steps to make sure future messages to our people regarding their choices on representation are always meaningful and respectful of their rights,” Bastian wrote.

The Delta chief went on to defend the company’s employee compensation, which he said has increased by 80% since 2008, adding that he is “unaware of any company our size that can make a similar statement.” He said the company has granted 10 pay increases over the last decade, and employees have seen their pay grow by more than 30% on average over the last five years.

He also disputed a claim by Sanders that ramp agents earn $9 per hour, which he called “simply wrong,” adding that starting salaries for the company’s ramp agents are nearly double that rate. He said flight attendants and ramp agents at the company can earn as much as $74,000 after 12 years of service.

A spokesperson for Sanders, in an emailed statement, repeated the claim that employees at Delta earn as little as $9 an hour, and said some make as little as $8.25.

“It’s time for Delta’s CEO to stop misleading, and stop discouraging workers from joining a union and pay all of them a living wage with good benefits,” the spokesperson said.

In his letter, Bastian touted the Delta’s profit-sharing plan, in which 15% of the company’s profits go to employees—a figure that has topped $1 billion for the last five years in a row. He also said the company’s 401(k) plan offers up to a 9% contribution from Delta, including a 3% company investment and a 6% dollar-for-dollar match when the plan member contributes 6%.

“When Delta faced bankruptcy a decade ago, we were the only airline that fought to retain our people's pension plans,” Bastian wrote. “It required an act of Congress, including a unanimous vote by the US Senate, to approve a rule change that preserved the accrued benefits our people had earned.”

The IAM filed charges with the US National Mediation Board (NMB) against Delta on May 15, alleging the carrier’s anti-union activities violate Delta employees’ right to unionize “free from interference, influence or coercion exercised by the carrier.”

“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 in a statement.

Ben Goldstein,