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.
It is important to recognise that this ideological U-turn did not happen in a vacuum, it was embedded the larger context of the history of popular US sentiment during the Cold War. From the fact that in his notes he explicitely selects the quote “Lenin awake” from the protest signs, it follows that at the time he still was a communist, just not a supporter of the Soviet intervention. His gradual shift towards republicanism coincides with the end of the big mass movements about civil rights and Vietnam in the 1970s, the beginning of the Reagan era, and the time when the Soviet economy began showing its first obvious signs of deterioration about a decade before the collapse. It came at a point in time where millions of other US-Americans and Europeans made a similar shift.
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.