Examine the basis for the groups that McCarthy found suspicious.
McCarthy and his House Unamaerican Activities Committee (HUAC) sought to find and "expose" Communists. McCarthy found this group suspicious because he believed that the Communist influence had infiltrated the "highest levels" of American government. The argument that he advanced to the American public who was already anxious about a Communist world was that American national security could not be guaranteed with a Communist influence and presence. This became the primary group that he found suspicious.
Along with this, McCarthy advocated the argument that the Communist influence could be seen in Hollywood. In one of the earliest examples of the "culture war" argument, McCarthy made the case to the American people that the values of the society could not be preserved with the nefarious element of Communism present in the American entertainment industry. From government, McCarthy and HUAC found many members of the Hollywood community as suspicious. The “Hollywood Ten" was a group of film writers and directors who appeared before McCarthy's group. They refused to "name names." While they found their professional and personal lives in tatters as a result of the Committee's suspicions, more Hollywood figures were called before the Committee and forced to either "name names" or suffer professional blacklisting and personal slander.
He wanted to expose communists therefore he found people with power the most likely to be suspicious. For example, he said many of Hollywood's best were communists.