What the House Un-American Activities Committee (HUAC) and Senator Joseph McCarthy had in common was that they both worried about communism subverting the United States and they were both willing to intimidate and bully people who they suspected of wrongdoing.  Both HUAC and McCarthy got a great deal of attention soon after WWII because Americans were very concerned about the potential spread of communism and the danger it posed.

Both HUAC and McCarthy were obsessed with the idea that communists were infiltrating the US.  McCarthy claimed that there were many communists in the State Department.  He claimed that communists were corrupting the Army.  HUAC investigated Hollywood moviemakers, believing that they were using the movies to try to win people over to communism.  They were both willing to bully people and to use questionable tactics.  McCarthy claimed to have lists of confirmed communists that he never shared with anyone.  HUAC would grill witnesses, trying to get them to admit to being communist and trying to get them to finger others.

Both HUAC and McCarthy fed off of popular fear of communism.  After WWII, many people worried that communism was spreading and becoming more threatening.  They saw the USSR take control over Eastern Europe.  They saw China become communist and they saw North Korea invade South Korea.  They saw the Soviet Union get atomic bombs.  They worried that communism would spread and so they supported demagogues who promised to prevent the spread by exposing alleged communists within the US.