Generating the Wealth of Nations 15: Union Membership in the United States
This chart shows union membership in the US as a percentage of the non-farm labor force. Until 1935, US law generally did not protect the right of workers to belong to unions. Indeed, until 1932, and the passage of the Norris-LaGuardia Act (http://en.wikipedia.org/wiki/Norris-Laguardia_Act), firms could, and some did, require workers to affirm that they were not union members, as a precondition of employment. After the National Labor Relations Act (Wagner Act) passed in 1935, union membership grew rapidly, peaking at over 35% of the non-farm labor force in the late 1950s.
(Click to enalrge. Source: Walton and Rockoff, An Economic History of the United States.)