(AP) — Elon Musk is inviting the United Auto Workers union to hold an organizing vote at Tesla’s factory in Fremont, California.

On Twitter Wednesday, Musk wrote that he invited the union to hold a vote at its convenience. “Tesla will do nothing to stop them,” he wrote.

The UAW wouldn’t comment Thursday but spokesman Brian Rothenberg pointed out that Tesla is fighting a U.S. National Labor Relations Board ruling from last year that found the company and Musk engaged in unfair labor practices in 2018, partly because of his tweets.

Musk’s recent tweets seem to defy the NLRB ruling, and are part of an escalating fight between Musk and federal regulatory agencies including the Securities and Exchange Commission and the National Highway Traffic Safety Administration.

Musk’s Wednesday tweet about the union came in response to one from Gene Simmons, the front man for the rock band Kiss, who was supporting Musk in a twitter spat with President Joe Biden over electric vehicle manufacturing.

Biden had tweeted on Tuesday about Ford and General Motors spending a total of $18 billion to create 15,000 jobs building electric vehicles. Musk replied that Tesla had created over 50,000 U.S. jobs building electric vehicles and is investing more than GM and Ford combined.

Simmons wrote that Musk had a “solid point” and questioned if Biden avoids mentioning Tesla because the company isn’t unionized. Unions have strongly supported Biden, and the UAW endorsed him in 2020.

Musk and his fan base have been upset with the president, who rarely mentions Tesla when he talks about electric vehicles and has left Musk off the invite list for electric vehicle events at the White House. Biden has long favored “good paying union jobs,” and workers at Tesla’s plants are not represented by a union.

A year ago, the labor relations board found that in a May 20, 2018 tweet, Musk unlawfully threatened employees with loss of stock options if they chose to be represented by the United Auto Workers union.

Board members ordered Tesla to make Musk delete the tweet and stop threatening employees with loss of benefits for supporting a labor organization.

Tesla also was required to post a notice addressing unfair labor practices at the 10,000-worker Fremont plant, and post a notice dealing with the tweet at all its facilities nationwide.

In April of 2021, Tesla appealed the NLRB ruling to a federal appeals court in New Orleans.

Among other things, the lengthy March 2021 NLRB ruling also ordered Tesla to reinstate an employee who was fired for union-organizing activity and to give him back pay.

Musk tweeted on May of 2018: “Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues and give up stock options for nothing? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare.”

The NLRB declined comment on Thursday.

Last month, California regulators sued Tesla Inc. alleging the electric car maker has been discriminating against Black employees who have been likened to monkeys and slaves at Fremont factory.