The union said in its letter that its labor contracts enshrine the right to a safe workplace, and that the presence of ICE or U.S. Border Patrol agents inside or in the immediate vicinity of properties constitutes “unusually dangerous conditions” that trigger employees’ right to refuse to work. The contracts also prohibit employers from allowing immigration agents onto their properties without a warrant, the letter said.

“We want to be clear,” the letter said. “If ICE or similar enforcement agents are present at or near your property, workers must be allowed to leave or refuse to report to work without reprisal.”