The Trump administration is attempting to fire a member of the Surface Transportation Board, as the agency considers a controversial merger proposed by Union Pacific and Norfolk Southern.

In an August 27 post to LinkedIn, STB member Robert Primus said that he had received an email earlier that evening from the White House telling him that his position had been terminated. Primus went on to describe the move as "troubling and legally invalid," and vowed to explore legal options to challenge his firing.

"Ironically, this comes at a time when the Board is considering significant pressing matters of critical importance to both our national freight rail network and supply chain that would directly affect large swaths of our manufacturing, agricultural, industrial and energy sectors," Primus added.

In a statement to CNBC, a White House spokesperson did not specify why exactly Primus had been terminated, stating only that he "did not align with the President's 'America First' agenda." Primus was first confirmed to the STB in the closing months of President Trump's first term, and later served as the board's chairman under the Biden administration. His term is set to expire in 2027.

The International Association of Sheet Metal, Air, Rail, and Transportation Workers — Transportation Division (SMART-TD) released a statement shortly after news first broke of Primus's firing, labeling it "nothing short of outrageous."

"Appointed bodies established through federal code are not designed to be erased at the whim of powerful corporate interests," the union said. "This action is unprecedented, unlawful in spirit, and reeks of direct interference from hedge funds and the nation’s largest rail carriers."

SMART-TD is one of several transportation unions that have spoken out against the proposed Union Pacific/Norfolk Southern merger since it was announced, over claims that it would jeopardize rail safety and lead to sweeping layoffs across the industry. If the deal goes through, UP would control 40% of the United States' rail freight.

The STB operates as an independent commission, with its members typically serving fixed terms. Under existing laws, the President can only remove board members for statutory causes, such as neglect of duty or committing a crime while in office. The Trump administration has faced legal challenges for attempts to remove officials at several other federal agencies that have similar rules as well. Most recently, Federal Reserve governor Lisa Cook sued Trump in federal court on August 28, claiming in her suit that he lacked sufficient cause to remove her from office.