Filing an application for an election requires a union to collect authorization cards from 50 percent of workers signed within the past year calling for an election -- which can be a challenge among a far-flung airline workforce.

The IAM called for the investigation after tweeting an image of a Delta flyer in an employee break room that suggested workers spend their money on a new video game system instead of on union dues.

Insulting. Without an IAM contract, many @Delta workers have to work 2-3 jobs just to survive. They have no time to play games. #DoBetterDelta pic.twitter.com/UVM0PR2fKC — Machinists Union (@MachinistsUnion) May 9, 2019

The airline has a campaign urging workers not to sign cards to call for a unionization vote, and has launched apps and a website called "Don't risk it. Don't sign it."

After the image of the flyer went viral, Atlanta-based Delta came under fire from U.S. Sen. Bernie Sanders, D-Vt., U.S. Rep. Alexandria Ocasio-Cortez, D-N.Y. and other union supporters.

In the complaint to the NMB, the union alleged Delta conducts surveillance of employees, and that managers photographed those participating in union activities and singled out and terminated union activists.

Delta argued that the board did not have jurisdiction to conduct an investigation because the union has not filed for an election. Delta also called the IAM’s claims of interference “completely baseless.”

“We respect our employees’ right to choose whether or not union representation is right for them, and we will continue to lawfully communicate with our employees and defend our federally protected right to educate them on the truth,” Delta said in a written statement.