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House Un-American Activities Committee
Communist Party in the US
Senator Joseph McCarthy
Blacklisting
The act of accusing a person of being a Communist, in the minds of many citizens, made them a Communist, and anyone who was associated with them got the same treatment. Artists were put on a "black list" in Hollywood for example, and were never hired to write, direct or act. Senator Joe McCarthy ("Tailgunner Joe") was the chief accuser.
Compare this to the girls in "The Crucible" simply accusing people of practicing witchcraft and having them imprisoned and hanged.