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.