During the colonial and revolutionary period of the US, most Americans practiced agriculture. The early US also had a substantial population of laborers, a group that included artisans, indentured servants and slaves. Many laborers resided in the major colonial cities of Philadelphia, New York and Boston, and at times these urban laborers organized against poor working conditions and low pay. The population of laborers in the US increased during the Industrial Revolution, as many Americans, including women and children, transitioned from agriculture to industrial jobs. Some unions existed by the start of the US Civil War in 1861, but by and large laborers remained unorganized.
The late 19th century, however, witnessed an increase in union activity. Early unions like the National Labor Union and the Knights of Labor emerged in the post-Civil War landscape but did not last long. On May 3, 1886, the Haymarket Affair turned public support against organized labor. After Chicago police fired into a group of striking laborers, an anarchist threw a stick of dynamite at the police and the explosion killed an officer. The incident did not spell doom for unions in America and Samuel Gompers helped the American Federation of Labor establish itself as one of the nation's largest labor unions.
Union activity in the US continued to increase, and in July 1935, unions received greater political protection when President Franklin Roosevelt signed into law the National Labor Relations Act. According to Roosevelt, the Act protected "the right of self-organization of employees in industry for the purposes of collective bargaining." It imposed a duty on employers to engage in collective bargaining with unions, and imposed a duty on workers to pay those negotiating unions under union security agreements. By 1947, however, some members of Congress proposed changes to the pro-union structure of the NLRA, and, over the veto of President Harry Truman, passed the Taft-Hartley Act. That Act amended parts of the NLRA, such as subjecting labor unions to claims of unfair practices and only allowing union shops in the absence of state law to the contrary.
Section 164 of the Taft-Hartley Act established the foundation for right-to-work laws by allowing states to prohibit union security agreements, or compulsory union membership. Within a year of the Taft-Hartley Act's passage, 12 states passed [PDF] right-to-work laws. Several more followed suit throughout the 1950's. As of December 2013, 24 states had enacted right-to-work laws. Michigan became the most recent state to pass a right-to-work law in December 2012. Michigan's law covers both public and private sector employees. The law does not prohibit employees from becoming members of labor unions, and instead allows employees the right to choose whether to refrain from union activity. The law creates a "right-to-work" by prohibiting union activity from being a required condition of employment.