McCarthyism
With basically all Americans hating communist there is bound to be some point where someone starts pointing fingers at others for being communist. This is exactly what Senator Joseph McCarthy did. He said he had a list of known communists however when asked if he could let others see the list he didn't say anything. However he would start accusing people of being communist spies. However, once he started accusing war vets he started to lose his validity and people stopped trusting him.
McCarthy may have never pointed out a real spy but he was right about there being spies because there were and they took classified information and shared it with the Soviet Union which allowed them to build an atomic bomb. Julius Rosenberg and Ethel Greenglass Rosenberg were the two spies who were eventually found guilty of sending this information and were sentenced to death.
And to prevent any communist from entering the country the Alien registration act was established which made it illegal to adovate or teach communism. It also required any foreigner over the age of 14 to file a comprehensive statement of their personal and occupational status and a record of their political beliefs to check to see if they were communist.
All of this ultimately led to the second red scare especially with McCarthy's actions, the fall of China to communism and the news that the Soviet Union had detonated an atomic bomb all led to great fear from even your neighbors that they were communists.