Final answer:
The flaw in McCarthy's speech is that few of his claims were supported by real evidence, exemplifying the technique known as McCarthyism.
Step-by-step explanation:
One flaw in McCarthy's speech is that few of his claims are supported by any real evidence. Senator Joseph McCarthy's methods during the Cold War era were characterized by accusations without the support of substantial evidence, a technique that came to be known as McCarthyism. His claims about a list of so-called 'card-carrying Communists' within the State Department were never substantiated, with numbers fluctuating and no concrete names presented. This approach fueled a red scare despite being baseless, which was politically advantageous for McCarthy despite the lack of veracity.