I obviously know very little about this situation but I only know vaguely of the fact that George Marshall was an early target of McCarthyism. I thought Marshall was mostly known for working to keep Communism from spreading beyond what we now call the Eastern Bloc, I thought McCarthy would be in support of that work?
Some background on McCarthy and the Red Scare is available in a previous answer I wrote here.
Specifically for George C. Marshall: the important thing to keep in mind is that Marshall was Truman's Secretary of State from 1947 to 1949, and Truman's Secretary of Defense from 1950 to 1951. While Secretary of State his Deputy was Dean Acheson (whom McCarthy relentlessly attacked as "Red Dean"), who succeeded Marshall as Secretary of State. McCarthy did not like the State Department at all, and saw it as a hotbed of Communist agents, homosexuals, and Northeastern liberal elites. So it's less that Marshall's role leading the department would come off well and more that Marshall would be tainted in McCarthy's eyes by the association with State.
The big strike of course was the fall of Beijing to the Communists in October 1949, giving the CCP a decisive victory in the Chinese Civil War. Marshall and Acheson in particular were singled out for "losing" China. On top of this, as Secretary of Defense through September 1951, Marshall was overseeing the US military at a point when the situation on the ground went incredibly sour for US forces with the Chinese intervention (which of course hearkened back to Marshall "losing" China in the first place). It also meant Marshall was Secretary of Defense, and supporting Truman, when Truman very controversially relieved General Douglas MacArthur of command for making controversial public statements in April 1951. It was a point when American public opinion was trending towards considering involvement in Korea to be a mistake, and when public support for Truman was in the mid 20s (it would bottom out at 22% in November, which is a historic record: no other president has gotten such a low public approval rating in Gallup polls).
From a domestic US perspective, the European Reconstruction Plan that got its unofficial name from Marshall was not really all that relevant by 1950-1952. McCarthy had actually voted for ERP in 1948, by the way, but in US domestic politics that had been far overshadowed by events in Asia. Some further background on the Marshall Plan can be found here.