Mandatory “captive audience” meetings in which companies argue against unionization are illegal, the National Labor Relations Board ruled in a case involving Amazon.com Inc., prohibiting one of employers’ most potent weapons against labor organizing campaigns.

Requiring workers to attend anti-union gatherings violates federal labor law protections that allow workers to freely choose whether, when, and how to participate in a debate about union representation—including refraining from doing so, the NLRB’s Democratic majority held in its Wednesday ruling.

“Ensuring that workers can make a truly free choice about whether they want union representation is one of the fundamental goals of the National Labor Relations Act,” Democratic Chair Lauren McFerran said in a statement. “Captive audience meetings—which give employers near-unfettered freedom to force their message about unionization on workers under threat of discipline or discharge—undermine this important goal.”

While the board majority handed unions a major victory with its captive audience ban, that win may be fleeting as the incoming Trump administration’s NLRB appointees will likely restore employers’ power to force workers to attend those gatherings.

The case stems from a series of mandatory anti-union meetings at Amazon’s Staten Island facility, where workers voted to unionize in 2022.

Amazon has been waging a high-profile battle against worker organizing. The company’s anti-union conduct has drawn rebukes from the NLRB. Amazon has recently taken to suing the agency.

The NLRB’s Wednesday decision overturned its 1948 ruling in Babcock & Wilcox Co., which permitted the mandatory anti-union gatherings. The board said its prohibition on those meetings will apply prospectively only, to accommodate the reliance that employers may have put on the 76-year-old precedent that it struck down.Mandatory anti-union gatherings interfere with workers’ organizing rights because they coercively demonstrate employers’ economic power by requiring attendance on pain of discipline or discharge, the NLRB said.

The meetings also provide a mechanism for companies to observe and surveil their workers as they listen to managers address the exercise of organizing rights, the board said.

Republican NLRB member Marvin Kaplan dissented from the ruling, saying the board’s ban on captive-audience speech won’t survive First Amendment scrutiny.

“Here, the conflict between the majority’s prohibition of captive-audience speeches and the Constitution is manifest and irreconcilable,” Kaplan said.