In 1947, Congress passed the Taft-Hartley Act which outlawed the closed shop, jurisdictional strikes, and secondary boycotts. It set up machinery for decertifying unions and allowed the states to pass more stringent legislation against unions such as right-to-work laws. Employers and unions were forbidden to contribute funds out of their treasuries to candidates for federal office, supervision was denied union protection, and the unions seeking the services of the National Labor Relations Board had to file their constitutions, by-laws, and financial statements with the U.S. Department of Labor. Their officers also had to sign a non-communist affidavit.
Source: U.S. Department of Labor