This Wisconsin senator gained fame by making claims that there were large numbers of Communists and Soviet spies and sympathizers inside the United States federal government.

The correct answer is Joseph McCarthy.

McCarthy's claims about communists being part of the federal government, the state department, and even the US military have been associated with the widespread fear of communism spreading in the US after World War II. Making claims like this, which became known as McCarthyism, resulted in hundreds of American citizens being brought before Congress through the House Un-American Activities Committee. From there, individuals could be punished for their ties to the communist party or with the Soviet Union. McCarthyism ended up ruining the careers/lives of several Americans during the 1950's.