Local 237 announced its endorsement of Democratic mayoral candidate William Thompson Thursday afternoon.
The union represents 24,000 people, from NYCHA employees to public hospital workers.
The president of Local 237 said he's known Thompson for years and thinks he'll give the city workers a fair contract.
"You've been treated unfairly. You've been ignored by the resident of City Hall. Let me let you understand now and today: That's going to end," Thompson said. "That will end the day I'm sworn in, January 1st of 2014."
The endorsement came one day after the United Federation of Teachers announced it's backing Thompson.