• The United Auto Workers filed federal labor charges against Donald Trump and Elon Musk for their comments applauding the practice of firing employees who threaten to strike.
• Under federal law, it is illegal to fire workers who threaten to strike.
• Trump has been vying for the labor vote in the presidential race against Vice President Kamala Harris, though the UAW has already endorsed the vice president.

The United Auto Workers union on Tuesday filed federal labor charges with the National Labor Relations Board against former President Donald Trump and billionaire Elon Musk for publicly applauding the practice of firing employees who threaten to strike.

"I look at what you do," Trump said to Musk during a two-hour interview Monday night on X, the social media platform Musk owns.

"You walk in, you say, 'You want to quit?' They go on strike," Trump said to Musk, who also is CEO of the EV car maker Tesla and of SpaceX.

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

Trump was referring to the 2022 gutting of Twitter staff after Musk took over the social media business and renamed it X.

It is illegal to fire workers who threaten to strike, because the right to strike is protected under federal labor law.

"When we say Donald Trump is a scab, this is what we mean," UAW President Shawn Fain said in a statement Tuesday on the new charges. "When we say Trump stands against everything our union stands for, this is what we mean."

Neither the Trump campaign nor Musk replied to CNBC's request for comment on the UAW action.

Trump's praise of union busting is notable because the Republican presidential nominee is currently fighting to win support from organized labor in a tight race against Vice President Kamala Harris.

The UAW, which represents more than 400,000 autoworkers, has already endorsed Harris. But another major U.S. labor union, the Teamsters, has yet to make an endorsement.

"Firing workers for organizing, striking, and exercising their rights as Americans is economic terrorism," Teamsters President Sean O'Brien said Tuesday in a statement to NBC News, responding to Trump's remarks.

In July, O'Brien delivered a speech at the Republican National Convention, which he said was intended to underscore that the union's powerful political endorsement was still available to whichever candidate pledged to champion workers' interests.

Musk is no stranger to labor battles. Tesla has clashed with union proponents for years, and Tesla workers remain without a union.

In 2021, the NLRB found that Tesla violated labor laws when it fired a union activist.

The board had made the same finding after Musk wrote on Twitter in 2018, "Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing?"

SpaceX has also been accused by the U.S. labor board of illegally firing eight employees, this time in retaliation over their internal, open letter criticizing Musk and his public conduct.

In response, SpaceX filed a suit claiming the NLRB's authority and administrative proceedings are unconstitutional.

This is a developing story. Please check back for updates.