In 1968, McCarthy triggered one of the most tumultuous years in American political history by forcing a national debate over Vietnam and the unbridled power of the presidency. Against formidable odds, he challenged Lyndon Johnson in the New Hampshire presidential primary election. Alienated voters flocked to his candidacy and LBJ, facing almost certain defeat, withdrew from the race.
(As it was, the Democrats nominated Hubert Humphrey, like McCarthy a U.S. senator and a Minnesotan, but unlike him a fallen liberal and party suck-up. The Republicans nominated a guy by the name of Richard Nixon, who kicked Humphrey's ass in the general election.)
I have no idea what kind of presidency McCarthy would have fashioned had his candidacy stayed alive, but I suspect it would have been as problematic as the man himself could be. That, however, is beside the point, especially when one considers the not dissimilar situation in Washington today: Is there no one to take on the Republican Party leadership in the midst of another conflict and topple a presidency with a war policy built on lies, deceit and hubris?
Asked once why he challenged the Johnson presidency, McCarthy answered by quoting from W.B. Yeat's "An Irish Airman Foresees His Death":
Nor law, nor duty bade me fight,
Nor public men, nor cheering crowds,
A lonely impulse of delight
Drove to this tumult in the clouds.