While studying chemistry at the University of California at Berkeley, he left the CP to join Max Shachtman’s Workers Party shortly before it changed to the Independent Socialist League in May 1949.
According to Robertson, the Workers Party gave up any perspective of reuniting with the Fourth International in 1948 and moved to the right under the pressure of the Cold War.
Along with Tim Wohlforth and Shane Mage, he was a leader of the Left Wing Caucus which developed in the YSL in early 1957 in opposition to Shachtman's plan to liquidate the ISL into what had become the Socialist Party-Social Democratic Federation.
Under the impact of the Hungarian Revolution of 1956, members of the Left Wing Caucus became convinced that the Stalinist bureaucracy was not a new ruling “bureaucratic collectivist” class, as Shachtman had insisted, but instead a brittle and unstable caste, as Trotsky had argued.
[3] In response to the formation of a Tibetan Brigade at the University of California at Berkeley in 1959, Robertson wrote a leaflet enthusiastically supporting the Chinese state’s suppression of what he considered to be a CIA-backed uprising in Tibet, reflecting the Young Socialist Clubs’ adoption of unconditional military defense of China, which they regarded as a deformed workers state, qualitatively similar to the USSR after it underwent its bureaucratic degeneration.
Robertson, Mage and Wohlforth opposed what they considered to be an uncritical embrace of Fidel Castro by the SWP leadership, which declared Cuba to be a workers state on a par with the early USSR.
The majority of the RT—which went on to found the Spartacist League—came to regard Cuba, like China, as a deformed workers state, qualitatively similar to the degenerated USSR.