A group of 33 US senators on Thursday called on Hyundai Motor Group and 12 other auto manufacturing companies not to interfere with the United Auto Workers’ ongoing efforts to unionize at their US plants.

Democrats Gary Peters, Ron Wyden, Dick Durbin and others signed a letter and sent it to top executives at Hyundai Motor Group, Tesla, Toyota, Volkswagen, Mercedes-Benz, BMW, Rivian and others, urging them to remain neutral in union organizing activities.

“We are concerned by reporting at numerous automakers that management has acted illegally to block unionization efforts,” the letter said. “These retaliatory actions are hostile to workers’ rights and must not be repeated if further organizing efforts are made by these companies’ workers. We therefore urge you all to commit to the implementation of a neutrality agreement at your manufacturing plants.”

“We believe a neutrality agreement is the bare minimum standard manufacturers should meet in respecting workers’ rights, especially as companies receive and benefit from federal funds related to the electric vehicle transition,” it added.

Addressing the illegal actions of automakers, it noted that Hyundai’s supervisors unlawfully banned pro-union materials in non-work areas outside of normal working hours. The same accusation was made by the UAW last month.

Hyundai at that time denied the accusation, saying, “Hyundai Motor Manufacturing Alabama team members may choose to join a union or not as is their legal right, and this has been true since our plant opened in 2005. We look forward to having a fair opportunity to present the facts through our participation in the legal process.”

Experts say the recent union organizing campaign puts the world’s third-largest carmaker in a tight spot, as it has already pledged to increase wages for its nonunionized workers at the Alabama plant.

“If the UAW unionizes workers at Hyundai’s production base, there would be additional increases in labor costs. This might raise the price of Hyundai cars sold in the US, which might result in losing price competitiveness in the all-important market,” said Kim Pil-su, a car engineering professor at Daelim University.

The UAW won labor contracts on wage hikes and cost-of-living salaries with Ford, General Motors and Stellantis in October last year after months of labor action. In November, Hyundai also announced it would raise wages by 25 percent for Alabama workers by 2028.