In June 1947 Congress passed the Taft-Hartley Act over the veto of President Harry S. Truman. The law consisted of a series of amendments to the Wagner Act of 1935. It retained the Wagner Act's framework of certification of unions through elections supervised by the National Labor Relations Board and its prohibitions against specified “unfair labor practices” through which employers attempted to prevent their workers from unionizing or tried to control unions, but it changed the thrust of the act by also outlawing such important union practices as closed shops, strikes in violation of contracts, mass picketing, secondary boycotts, and other actions of solidarity. It also banned union contributions to political candidates, forbad employees of the government from striking, permitted states to outlaw union security agreements, and authorized the president to seek a court injunction forbidding for eighty days any strikes that might affect national health or safety. Its most dramatic innovation was a requirement that no union could appeal to the protections of the law or the services of the NLRB unless its elected officers all signed affidavits stating that they were not members of the Communist Party.

Truman's veto message contended that the bill would “reverse the basic direction of our national labor policy, inject the government into private economic affairs on an unprecedented scale, and conflict with important principles of our democratic society.”