The flyer was criticized by many pro-union advocates and politicians on social media. Sanders and other senators wrote letters to Bastian calling on him to "end Delta's anti-union tactics."
Bastian responded to Sanders that he agrees that the flyer, featured in a photo that went viral on Twitter, “was poorly crafted and not an appropriate communication to our people.”
"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 in the letter.
The International Association of Machinists union (IAM) has been seeking to organize Delta flight attendants and baggage handlers and is criticized in Delta flyers.
Last week, the IAM filed a complaint with the National Mediation Board against Delta over what it called "anti-union tactics."
In the letter to Sanders, Bastian wrote that Delta's employees "are the best-performing, best-compensated aviation professionals in our industry."
He wrote that Delta employees on average have had base pay increases totaling more than 30 percent over the last five years, and that ramp agents' starting salaries are nearly double the $9 per hour cited by Sanders.
However, some jobs for contractors like Delta Global Services that handle work for Delta and other airlines pay less. A search on the Delta Global Services job board showed job postings for ramp agents as low as $8.25 or $8.50 an hour. Pay rates are higher for some locations.
Bastian wrote that Delta flight attendants and airport agents earn as much as $74,000 at top scale after 12 years of service, while mechanics can earn as much as $121,000 annually. The company also pays hefty profit sharing bonuses to employees.