How did McCarthy finally lose power?

Lack of evidence that the Army had been infiltrated by Communists was one factor, as were his tactics, which produced revulsion in his colleagues, as well as the public at large.

His fall from influence was precipitated by his censure by the Senate, which was passed in December 1954. Originally charged with 46 counts of violating senate rules and procedures, he was ultimately cited on the count of "contempt of the Subcommittee on Rules and Administration," which had called him to testify in 1951 and 1952. Influential senators had been opposing him from as early as 1950, however.

Interestingly, John F. Kennedy was the only Democrat not to vote in favor of the measure. He was in the hospital at the time. But his younger brother Robert had been a prominent staff attorney working for McCarthy's committee. The elder Kennedy never indicated which way he would have voted if he had been able to attend.