The correct answer to this open question is the following.
American citizens might have accepted Senator McCarthy's statements as facts, even his shocking truth about the State Department because in those years of the Cold War, American people lived in fear and misinformed about the way Communism was spread in different countries of the world.
Those were the years in which the Soviet Union and Communism were the worst enemies of the United States. Mass media covered events and reported the news that instilled fear in the American people about a possible intromission of the Sob¿viet Union in American affairs. That is why Wisconsin Republican Senator Joseph McCarthy had credibility at first, but because he could not prove anything, his political career ended soon and was considered a lier. However, he ruined the reputation of many people in the federal government and the military.