Why was Joseph McCarthy so dangerous?

Answer 1

McCarthy was so powerful because so many different agencies and individuals took part in McCarthyism-related operations. The public was so frightened by the supposed (and real) threat of communists undermining US security that they supported anyone who seemed to be fighting it.

Answer 2

McCarthy claimed that there were known communists in the State Department. He never produced any proof and he accused people and ruined many careers merely by "hinting" that certain people were communists or communist sympathizers. He held investigations and hearings that proved nothing. Presidents Truman and Eisenhower so feared his influence with the public that they did nothing to stop him. When McCarthy attacked the US Army as a hotbed of communist activity, the Senate finally censored him and he died a broken politician.