"The Taft–Hartley Act amended the 1935 National Labor Relations Act (NLRA), prohibiting unions from engaging in several "unfair labor practices." Among the practices prohibited by the act are jurisdictional strikes, wildcat strikes, solidarity or political strikes, secondary boycotts, secondary and mass picketing, closed shops, and monetary donations by unions to federal political campaigns. The NLRA also allowed states to pass right-to-work laws banning union shops. Enacted during the early stages of the Cold War, the law required union officers to sign non-communist affidavits with the government. "
https://en.wikipedia.org/wiki/Taft%E2%80%93Hartley_Act