(NewsNation) — The United Auto Workers union said Tuesday it has filed federal labor charges against former President Donald Trump and billionaire Elon Musk, accusing the pair of threatening and intimidating workers.

The move comes after a conversation on X Monday night between Trump and Musk, who advocated for the firing of striking workers, which is illegal.

“They go on strike and you say, ‘that’s OK, you’re all gone,'” Trump said. “You’re all gone, every one one of you is gone.”

Trump appeared to also be referencing Musk’s decision to gut the staff at X, then known as Twitter, after buying the company in 2022. Musk faced lawsuits from employees following the mass layoffs and has also faced accusations of illegally firing workers at SpaceX.

The charges come as Trump has spoken out against UAW President Shawn Fain, while in contrast, Teamsters President Sean O’Brien spoke at the Republican National Convention.

The UAW has endorsed Vice President Kamala Harris in the 2024 presidential race, as have other unions including the AFL-CIO, the Service Employees International Union, the hospitality workers union UNITE HERE, the American Federation of State, County and Municipal Employees, and the American Federation of Teachers.

The Teamsters union has yet to officially endorse either candidate.

Under federal law, workers can’t be fired for going on strike, and using the threat of firing striking workers is illegal.

“Donald Trump will always side against workers standing up for themselves, and he will always side with billionaires like Elon Musk, who is contributing $45 million a month to a Super PAC to get him elected,” Fain said in a news release. “Both Trump and Musk want working-class people to sit down and shut up, and they laugh about it openly.”

It’s up to the National Labor Relations Board to investigate the charges brought by the UAW. If it finds Trump or Musk violated labor laws, it could try to resolve the case or sue them on behalf of the UAW, which could potentially lead to fines.