On July 5, 1935, President Franklin D. Roosevelt signed the National Labor Relations Act into law. More commonly known as the Wagner Act, after Robert F. Wagner, the US senator who introduced the bill, the law recognized the right of workers to engage in collective bargaining, to be represented by a union of their choice, and to take strike action.
The law was a blow against the “open shop” and company unions favored by employers, and was the second attempt at regulating labor practices following a Supreme Court’s ruling that declared unconstitutional the National Industrial Recovery Act...But while the act gave certain protections to organizing workers that contributed to a growth of union membership throughout the 1930s, it also sought to restrain the growing militancy of the working class.
Roosevelt wagered that if the main trade unions, the American Federation of Labor (AFL) and the young Congress of Industrial Organizations (CIO), were promoted as the legitimate representatives of the working class, they could be absorbed into the “New Deal Coalition” and serve as a barrier to the militant struggles of the working class that had taken on near-insurrectionary proportions. The Wagner Act sought to move labor disputes off the streets and put them in the arena of federal arbitration boards.
The previous year had seen a wave of city-wide strikes, including the San Francisco longshoremen's strike, the Toledo Auto-Lite strike, and the Minneapolis truck drivers’ strike...The Wagner Act also granted the president the right to bring a strike to an end in the event that it was found to “imperil the national health or safety.”
http://www.wsws.org/articles/2010/jul2010/twih-j05.shtml#50