McCarthyism
....attempts to introduce legislation or apply existing laws to help to protect the United States from the perceived threat of Communist subversion.
The Alien Registration Act or Smith Act of 1940 made it a criminal offense for anyone to "knowingly or willfully advocate, abet, advise or teach the […] desirability or propriety of overthrowing the Government of the United States or of any State by force or violence, or for anyone to organize any association which teaches, advises or encourages such an overthrow, or for anyone to become a member of or to affiliate with any such association". Hundreds of Communists and others were prosecuted under this law between 1941 and 1957. Eleven leaders of the Communist Party were charged and convicted under the Smith Act in 1949. Ten defendants were given sentences of five years and the eleventh was sentenced to three years. All of the defense attorneys were cited for contempt of court and were also given prison sentences. In 1951, twenty-three other leaders of the party were indicted, including Elizabeth Gurley Flynn, a founding member of the American Civil Liberties Union. By 1957 over 140 leaders and members of the Communist Party had been charged under the law.[32]
Described by scholar Ellen Schrecker as "the McCarthy era's only important piece of legislation,"[33] the McCarran Internal Security Act became law in 1950 (the Smith Act technically predated McCarthyism). However, the McCarran Act had no real effect beyond legal harassment. It required the registration of Communist organizations with the Attorney General and established the Subversive Activities Control Board to investigate possible Communist-action and Communist-front organizations so they could be required to register. Due to numerous hearings, delays and appeals, the act was never enforced, even with regard to the Communist Party of the United States itself, and
the major provisions of the act were found to be unconstitutional in 1965 and 1967.[34]
http://en.wikipedia.org/wiki/McCarthy_Era#Laws_and_arrests