The endorsement would not only boost Thompson's underfunded campaign by supplying campaign volunteers, it would also open the door for other city unions, like the UFT and Teamsters Local 237 -- both of which are now negotiating contracts -- to at least use the threat of defection at the bargaining table, if not actually defect.
Over the past year, union bosses say, the relationship between DC 37 and the Mayor has soured. The union bought subway ads this spring saying the city wastes $9 billion a year privatizing jobs city workers could do.
Bloomberg took a blow from labor last month when the union-backed Working Families Party endorsed Thompson.
Thompson and Bloomberg are now running about neck-and-neck in union endorsements, yet another indicator that there's more game in the mayoral race than predicted.