What factors led to mccarthy’s eventual downfall?

The main factor is that people stopped fearing communism. What also helped was that he was slowly becoming insane, going as far as saying that the United States army had communists running it deep within its ranks. People eventually realized that the USSR was not really that big of an issue and that the US was far superior when it came to firepower and wealth and they realized that McCarthy was not the sharpest tool in the shed.