He visitied USSR many times and knew Russian well. He rejected communism after he saw the Warsaw Pact intervention in Czechoslovakia. The intervention took less than 300 hundred lives on both sides, but McCarthy choose to support republicans, a fascist regime responsible for thousands of victims all over the World.
Edit: Wikipedia says: “he distanced himself after making visits to the Soviet Bloc, which led to him becoming a conservative Republican.” but it seems that this particular intervention changed his mind, not the earlier multiple visits in Soviet Union. Wikipedia is biased as fuck.
That seems probable. Additional conjecture is that, since we are attracted by unknown ideas, new for us, it additionally helped McCarthy to follow this right-wing trend of that sad era.