All major unions I'm aware of are activists for the Democratic Party. I spend a lot of time researching this area. It doesn't come as any surprise once you have a grasp of the labor movement.
Two wonderful site to track PAC donations are
AFL CIO President, John Sweeney, is a Member of Scialist International and Democratic Socialists of America
Once the Union leadership and their political beliefs are understood it's no surprise they are linked to the Democratic Party.