A major US auto workers union filed federal labor charges Tuesday against Donald Trump and Elon Musk, accusing the billionaires of illegally intimidating workers during a rambling and highly publicized conversation on X.

The Republican presidential hopeful and Musk, who has endorsed him, exchanged a checklist of radical conspiracy theories as they chatted on the social network Monday night.

At one point the conversation veered into Musk's time at the helm of his multiple companies.

"I mean, I look at what you do," Trump said to Musk.

"You walk in, you say, 'You want to quit?' They go on strike," Trump said.

"I won't mention the name of the company, but they go on strike and you say, 'That's OK, you're all gone. You're all gone. So, every one of you is gone.'"

Musk carried out massive layoffs after his acquisition of Twitter (now X) was finalized in October 2022, and he announced in April the dismissal of more than 10 percent of Tesla's global workforce, amounting to at least 14,000 people.

"Under federal law, workers cannot be fired for going on strike, and threatening to do so is illegal under the National Labor Relations Act," said the United Auto Workers (UAW), which has endorsed Democratic presidential candidate Kamala Harris after previously backing Joe Biden.

"When we say Donald Trump is a scab, this is what we mean. When we say Trump stands against everything our union stands for, this is what we mean," said UAW President Shawn Fain.

"Both Trump and Musk want working class people to sit down and shut up, and they laugh about it openly," Fain said. "It's disgusting, illegal, and totally predictable from these two clowns."

Musk owns the social network X, SpaceX and electric vehicle maker Tesla, among other companies. He has been criticized in the past for obstructing the unionization of his employees around the world.

Tesla has been at odds with unions in Sweden and Norway for a few months.

Several complaints have also been filed in California by former SpaceX workers who say employees who tried to protest job conditions were retaliated against.