Excellent chart from the BLS on the union affiliation of U.S. wage and salary workers.
…unlike in the private economy, a public union has a natural monopoly over government services. An industrial union will fight for a greater share of corporate profits, but it also knows that a business must make profits or it will move or shut down. The union chief for teachers, transit workers or firemen knows that the city is not going to close the schools, buses or firehouses.
This monopoly power, in turn, gives public unions inordinate sway over elected officials. The money they collect from member dues helps to elect politicians who are then supposed to represent the taxpayers during the next round of collective bargaining. In effect union representatives sit on both sides of the bargaining table, with no one sitting in for taxpayers.
(click here if charts are not observable)