The United Auto Workers Union said on Tuesday it has filed charges with the National Labor Relations Board against Donald Trump and Tesla CEO Elon Musk over attempts to threaten and intimidate workers.

The action came after Musk and Trump held a two-hour conversation on social media platform X on Monday night, during which Trump complimented Musk’s ability to cut costs by saying he would not tolerate workers going on strike.

“You’re the greatest cutter,” Trump said during the conversation. “I mean, I look at what you do. You walk in, you just say: ‘You want to quit?’ They go on strike – I won’t mention the name of the company – but they go on strike. And you say: ‘That’s okay, you’re all gone.’”

Musk chuckled but did not respond to Trump’s comments, making it harder for the NLRB to find him liable for making illegal threats to workers at his companies, said Wilma Liebman, chair of the NLRB under former President Barack Obama.

Under federal law, workers cannot be fired for going on strike, and threatening to do so is illegal under the National Labor Relations Act, the UAW said in a statement.

It is unclear whether the NLRB would take action against Trump for his comments, but the UAW jumped on his remarks as it continues to rally behind Democratic presidential candidate and Vice President Kamala Harris and encourage its nearly 400,000 workers to vote for her over Trump, especially in battleground states like Michigan that could determine who wins the White House in November. The UAW is based in Michigan and has many members in that state.

The UAW endorsed Harris at the end of July. She met with union officials and workers last week near Detroit.

UAW President Shawn Fain and Trump have exchanged barbs in the past.

“Both Trump and Musk want working class people to sit down and shut up, and they laugh about it openly. It’s disgusting, illegal, and totally predictable from these two clowns,” Fain said in a statement on Tuesday.

The Harris and Trump campaigns did not immediately respond to requests for comment. Musk also did not respond to a request for comment.

Trump has called for the union leader to be fired, saying he is responsible for U.S. auto manufacturing becoming weaker.

UAW members in Michigan tend to side with Democrats, but pro-Trump workers have organized their own rallies in recent weeks to show their support for the former president.

Sending a message

The NLRB has limited power to punish unlawful labor practices. In cases involving illegal threats, the board can order employers to cease and desist from such conduct and to post notices in the workplace informing workers of their rights. Unions can also use favorable rulings from the NLRB to engage workers they are trying to organize.

“It’s trying to expose more than anything politically what Donald Trump is about in terms of workers, and Musk as well,” former NLRB head Liebman said, referring to the UAW’s action on Tuesday. “Everyone knows the NLRB remedies are toothless to start with, but it’s not so much for the remedy as for sending both a political message and an organizing message.”

Fain filed separate complaints against Musk and Trump with the NLRB, claiming both men had made statements suggesting they “would fire employees engaged in protected concerted activity, including striking.” The complaints did not provide further detail.

The UAW led a six-week strike against Detroit’s Big Three automakers last autumn, in which workers at Ford Motor, General Motors and Jeep maker Stellantis walked picket lines across the country.

The union won record contracts, which included a 25% general wage increase over the life of the agreement.

Musk and the NLRB

Musk, who has endorsed Trump for president, has had numerous run-ins with the labor board. His rocket company SpaceX is currently challenging the entire structure of the agency in a pair of pending lawsuits. Those cases stemmed from NLRB complaints accusing SpaceX of firing engineers who were critical of Musk and forcing employees to sign severance agreements with unlawful terms.

In March, a U.S. appeals court upheld an NLRB decision that said Musk illegally threatened Tesla employees by tweeting in 2018: “Nothing stopping Tesla team at our car plant from voting union … But why pay union dues & give up stock options for nothing?”

Tesla is separately facing allegations from the board that it illegally discouraged unionizing at a Buffalo, New York, plant. Last year, an appeals court threw out a labor board decision that said Tesla broke the law by barring factory workers from wearing UAW T-shirts.

Lawyers at the labor board will investigate the union’s claims and decide whether to issue formal complaints against Tesla and the Trump campaign. Those cases would be heard by administrative judges whose decisions can be reviewed by the five-member labor board and then federal appeals courts. The process often takes years.

—Nora Eckert and Daniel Wiessner, Reuters

Akash Sriram, Jarrett Renshaw, Tim Reid and Ben Klayman contributed to this report.