Final answer:
Wisconsin Senator Joseph McCarthy falsely claimed to have a list of 205 Communists in the State Department, sparking the second Red Scare through his tactics, which came to be known as McCarthyism.
Step-by-step explanation:
The U.S. Senator who claimed to have the names of 205 communists working at the State Department, which led to the second Red Scare, was Joseph McCarthy. He gained notoriety for his unsubstantiated claims during a speech in Wheeling, West Virginia, in February 1950. McCarthy's accusations were found to be false as the numbers he claimed continually shifted, and he never produced any actual list or conclusive evidence. His tactics, which involved making allegations without proof, later became known as McCarthyism. Despite the fact that spying for the Soviet Union did occur, McCarthy's sensational allegations were largely considered a political strategy to fuel fears of communism and to bolster his own political position.
His activities contributed significantly to the era of paranoia and repression that characterized the Cold War era in the United States.