As the legislative representative for his local union, Gabe Christenson, a longtime freight railroad conductor, worked hard to help elect Joe Biden president in 2020. “I have shirts of me campaigning — blue-collar Biden shirts,” he said. “I knocked on doors for him for weeks and weeks.”

But since Monday, when President Biden urged Congress to impose a labor agreement that his union had voted down, Mr. Christenson has been besieged by texts from furious co-workers whom he had encouraged to support the president. “I’m trying to calm them down,” he said. …

Several union members and local officials said they had urged co-workers who had previously supported Donald Trump to back Mr. Biden, arguing that they he would be friendlier to labor. They said that these co-workers had reached out to complain about what they saw as Mr. Biden’s about-face since Monday, though it was unclear how many of these union members had voted for the current president.

“Many Trump voters calling me out for endorsing Biden,” said Matthew A. Weaver, a carpenter with rail maintenance employees union, said by text Tuesday night. Mr. Weaver previously worked as an official for his union in Ohio.