Let's take a look at the opening lines of the Welch-McCarthy showdown. It's Senator Sleaze who gets the ball rolling:

[…] in view of Mr. Welch's request that the information be given once we know of anyone who might be performing any work for the Communist Party, I think we should tell him that he has in his law firm a young man named Fisher whom he recommended, incidentally, to do work on this committee, who has been for a number of years a member of an organization which was named, oh, years and years ago, as the legal bulwark of the Communist Party, an organization which always swings to the defense of anyone who dares to expose Communists. (McCarthy.78)

This is McCarthy's opening salvo to get Welch to stop his effortless shellacking of Roy Cohn on the stand. While McCarthy had made a cursory attempt to stop the line of questioning by claiming the FBI already had the information on where these hidden Communists were, it was literally one line. Then it was right to the smear.

Welch was probably thinking, "Make my day."