(Reuters) - The publisher of conservative online magazine The Federalist has moved to overturn a National Labor Relations Board ruling that said its owner violated federal labor law by tweeting that he would send employees "back to the salt mine" if they unionized, saying it was satirical and a valid expression of free speech.

FDRLST Media LLC, represented by the conservative New Civil Liberties Alliance, said Ben Domenech's 2019 tweet was protected by the First Amendment of the U.S. Constitution because it was clearly intended as a joke and was posted on his personal Twitter account, in a brief filed with the 3rd U.S. Circuit Court of Appeals on Monday.

But most of FDRLST's brief focused on its argument that the NLRB never had jurisdiction over the case because it was based on a complaint filed by a "random" member of the public, and not an employee of the company or anyone else "aggrieved" by Domenech's words.

"Allowing random, unaggrieved people to turn the government's enforcement machinery loose on a company would interfere with employer–employee relations if the employees — as here — do not agree with the random outsider's perspective," the company's lawyers wrote.

An NLRB spokesman did not immediately respond to a request for comment.

In June 2019, unionized employees of Vox Media, a digital media network that carries stories produced by other outlets including the Federalist, staged a walkout that received significant media attention.

On the day of the walkout, Domenech tweeted: "FYI @fdrlst first one of you tries to unionize I swear I'll send you back to the salt mine."

Joel Fleming, a securities litigator with Block & Leviton in Boston, filed a complaint with the NLRB, claiming the tweet would interfere with the rights of The Federalist's employees to unionize under the National Labor Relations Act.

FDRLST maintained that the tweet was a joke, and the fact that Domenech posted it publicly on Twitter showed that it was not directed at the website's employees. The company also said the NLRB did not have the ability to take up Fleming's complaint because he had no connection to The Federalist.

An administrative law judge ruled against the company, saying the tweet sent the message that working conditions would worsen or employee benefits would be jeopardized if employees unionized.

The NLRB in November agreed, saying the tweet was plainly directed at The Federalist's employees and that whether an employer's comments involving unionizing violated NLRA does not turn on the employer's motive, but on whether the comments tend to restrain workers' rights to organize.

The board ordered FDRLST to direct Domenech to remove the tweet from his personal account.

On Monday, the company's lawyers told the 3rd Circuit the board was wrong to summarily reject its claim that Fleming had no standing to file a complaint. They called Fleming an "officious interloper," and said allowing the NLRB decision to stand would lead to unjust results.

"Congress did not enact the NLRA so that anyone could wield (the law) as a sword against business competitors or ideological adversaries," the company said.

The case is FDRLST Media LLC v. NLRB, 3rd U.S. Circuit Court of Appeals, No. 20-3434.

For FDRLST: Aditya Dynar of the New Civil Liberties Alliance

For the board: Kira Vol