Reunification with the Social Democratic Federation was long a goal of Norman Thomas and his associates remaining in the Socialist Party. As early as 1938, Thomas had acknowledged that a number of issues had been involved in the split which led to the formation of the rival Social Democratic Federation, including "organizational policy, the effort to make the party inclusive of all socialist elements not bound by communist discipline; a feeling of dissatisfaction with social democratic tactics which had failed in Germany" as well as "the socialist estimate of Russia; and the possibility of cooperation with communists on certain specific matters." Still, he held that "those of us who believe that an inclusive socialist party is desirable, and ought to be possible, hope that the growing friendliness of socialist groups will bring about not only joint action but ultimately a satisfactory reunion on the basis of sufficient agreement for harmonious support of a socialist program."
The Socialist Party and the SDF merged to form the Socialist Party-Social Democratic Federation (SP-SDF) in 1957. A small group of holdouts refused to reunify, establishing a new organization called the Democratic Socialist Federation. When the Soviet Union led an invasion of Hungary in 1956, half of the members of Communist Parties around the world quit; in the U.S., half did, and many joined the Socialist Party.