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

It is unclear whether the NLRB would take action against Trump for his comments on Monday during a two-hour conversation with Musk that was broadcast on social media platform X.

The UAW seized upon Trump's remarks as the union rallies behind Democratic presidential candidate and Vice President Kamala Harris and encourages its nearly 400,000 workers to vote for her over Trump. The issue is especially pertinent in battleground states like Michigan which could determine who wins the White House in November. The UAW endorsed Harris at the end of July.

"You're the greatest cutter," Trump said to Musk during Monday's conversation, complimenting the CEO's ability to cut costs by saying he would not tolerate workers going on strike. "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. 

After the union's action on Tuesday, Musk criticized UAW President Shawn Fain in a social media post, alluding to two past union presidents who went to prison for bribery and corruption. "Based on recent news, it looks like this guy will join them!" Musk said.

A court-appointed monitor of the UAW is investigating several union executives, including allegations that Fain retaliated against a member of his board when the person did not take actions that would have benefitted Fain's domestic partner and her sister. The UAW could not immediately be reached to comment on Musk's response.

Trump campaign officials said his pledged 100% tariff on Chinese imports would strengthen the industry, while Harris' EV policies are hurting American manufacturing.

"This frivolous lawsuit is a shameless political stunt intended to erode President Trump's overwhelming support among America's workers," Trump campaign senior adviser Brian Hughes said in a statement.

The Harris campaign did not immediately respond to requests for comment after the UAW's action.

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.

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.

In the 2020 presidential race, 62% of Michigan households with a union member voted for President Joe Biden, helping him win the state, according to Edison Research. By contrast, union households split 53% to 40% for Hillary Clinton in 2016, when she narrowly lost the state and the national race. 

Sean O'Brien, president of the Teamsters, another key labor group, said of Trump's comments, "Firing workers for organizing, striking, and exercising their rights as Americans is economic terrorism."

The Teamsters have traditionally endorsed a candidate after party conventions. O'Brien spoke at the Republican National Convention in Milwaukee in July.

The union had also requested a speaking opportunity at the upcoming Democratic National Convention but it had not heard back, spokesperson Kara Deniz said. It had also invited Harris to a roundtable but had not heard back.

SENDING A MESSAGE

The NLRB has limited power to punish unlawful labor practices and the process often lasts years. 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.

"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," former NLRB head Liebman said, referring to the UAW's action on Tuesday.

Fain filed separate complaints with the NLRB against the Trump campaign and Tesla citing Trump and Musk as the employers' representatives, 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 NLRB has jurisdiction over the Trump campaign as an employer, but not Trump himself.

The UAW led a six-week strike against Detroit's Big Three automakers last autumn, before winning record contracts.

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.