Silicon Valley entrepreneurs are famously resistant to labor unions, and Tesla CEO Elon Musk is no different. But the billionaire mogul is now facing a unionization push by workers at the California factory that makes Tesla electric vehicles — and Musk is fighting back in the press.

In a Medium post published Thursday, Tesla factory worker Jose Moran explained why he reached out to the United Automobile, Aerospace and Agricultural Implement Workers of America (UAW) to unionize the Tesla plant. Moran said that mediocre pay, unsafe working conditions, and long hours formed part of the basis for the union drive.

“Most of my 5,000-plus coworkers work well over 40 hours a week, including excessive mandatory overtime,” Moran said. “The hard, manual labor we put in to make Tesla successful is done at great risk to our bodies. Preventable injuries happen often.”

Musk responded that same day to Moran in harsh terms in a direct-message conversation with Gizmodo. He described Tesla as “union-neutral,” criticized Moran’s post as “morally outrageous,” and said that Tesla’s understanding “is that this guy was paid by the UAW to join Tesla and agitate for a union. He doesn’t really work for us; he works for the UAW.”

On Friday morning, UAW replied with a statement denying Musk’s accusation, calling it “fake news.”

“This is not the first time we have been the target of a professional union organizing effort such as this,” a Tesla spokesperson said in a statement provided to VICE News. “The safety and job satisfaction of our employees here at Tesla has always been extremely important to us.”

On a Friday-evening phone call with reporters set up by the UAW, Moran said that Musk’s response “goes to show what respect they have for workers’ opinions. The UAW is here for support.”

Moran said he had worked for Tesla since 2012; the company did not respond to a request for clarification on whether it stands by Musk’s accusation that Moran does not work for Tesla.

In his Gizmodo interview, Musk said that Tesla workers make more than the starting salary for UAW union members, with the opportunity to get Tesla stock grants. He added that the UAW was responsible for the closure of the former Toyota-GM plant that is now Tesla’s Fremont, California facility.

That plant, dubbed NUMMI, was a UAW shop set up in the early 1980s as a joint GM-Toyota venture. It closed in 2010 after Toyota determined that it was too costly to keep making cars there. At the time, Toyota said that the UAW wasn’t to blame, pointing instead to the high cost of living in the San Francisco Bay Area. Many UAW workers, however, were furious with union leaders for secretly negotiating a $278 million severance package with NUMMI management.

Speaking with reporters, Moran said that there are a large number of ex-NUMMI employees working at the Tesla facility, and that there are “still some hard feelings” related to the NUMMI plant closure. Moran himself is an ex-NUMMI factory worker, and he says that he got the Tesla job after a recruiter reached out to him.

However, Moran said, the response from his coworkers has largely been “positive,” and no one (including management) has yet given him any pushback. Members of his Facebook group, which now includes about 200 other Tesla factory workers, are also concerned about safety issues and the company’s rigorous work schedule.

In a letter sent to employees last year, Musk urged staff to cut expenses and maximize efficiencies in order to “throw a pie in the face of all the naysayers on Wall Street” who criticized the company’s high production costs. The demanding CEO once said he was “extremely disappointed” in a Tesla employee who missed a corporate event for the birth of his child, according to a 2015 book by Bloomberg reporter Ashlee Vance.

Moran, who works in a part of the factory that produces the Tesla Model S luxury sedan, described a “12-hour, six-day-a-week” work environment that takes a heavy toll on workers’ bodies. He says that he and other workers want a UAW chapter in large part to better negotiate worker protections.