During the Cold War the US went through a period called the Red Scare, where there were hunts for suspected communists.
In the 40s-50s Americans became anxious about communists invading the home front. There were some basis for these fears. The USSR had long attempted to carry out espionage inside the US, sometimes aided by US citizens.
In response to these fears President Truman issued the 1947 Executive Order 9835, also called the loyalty order. It mandated that all federal employees be screened to ensure they were sufficiently loyal to the government.
This lead to an era of paranoia where witch hunts were conducted trying to out suspected communists. Thousands were accused of being communist sympathizers. Many were alienated them from friends and family, hounded by law enforcement, and fired from their jobs. A small number of the accused may have had some aspirations against the United States but the vast majority were falsely accused or had done nothing more than exercise their democratic right to join a political party.