Why did the fear of communism at home reach such great proportions?

The fear of communism in the United States was fueled by Senator Joseph McCarthy and the proceedings of the  House Un-American Activities Committee.

Joseph McCarthy was a US Senator that claimed to have a list of communists that were involved in the US State Department in the 1950's. The shock of this accusation made American citizens more paranoid about the government. Along with this, McCarthy began accusing other politicians, celebrities, and even the US military of having communist influences.

This rise in McCarthyism helped increase the amount of proceedings in the House Un-American Activities Committee (HUAC). HUAC was run by the US Congress. Congress would call in citizens to question their ties to communism or the Soviet Union. These hearings ruined the lives of hundreds of American citizens and only caused more paranoia among US citizens after seeing them on TV.