Since you say Socialist rather than socialist, I assume you mean a president from the Socialist Party. In all likelihood, getting one requires a POD way before 1945 but if we just wave our hands and make Norman Thomas, the six-time Socialist Party candidate somehow become President, I would say that of all the presidential candidates in 1948 (Truman, Dewey, Thurmond, Wallace) he was the *only* one who was really critical of the Soviet Union during World War II.
And of course the Soviets and their American fan club reciprocated. Israel Amter wrote in *The Communist* in 1942:
"Mussolini was a “socialist,” Laval was a “socialist,” Norman Thomas, too, is a "Socialist." He offers the world only one kind of peace—-the peace of a Hitler, a Mussolini, a Laval...Norman Thomas, fifth columnist and spearhead of fascism, still has access to the radio and spews forth his traitorous program. It is a distinct disservice to our country to allow this worker for fascism to use the air in order to spread disunity and hatred for our allies. Let us not allow ourselves a repetition of the fate of France, where the fascists were permitted to carry on their work. Let us rather adopt the methods of the Soviet Union..."
http://www.unz.org/Pub/Communist-1942jun-00450