Sartre was a leftist thinker throughout his entire life, and after World War II, he moved more and more to the left, expressing himself in increasingly difficult publications, and often in a language accessible only to a select few.
He rejected the idea of class and attempted to strip away the layers of bourgeois values imposed by the capitalistic society in which he lived. His major goal, politically, was to have a country in which total freedom existed — a true democracy, not a dictatorship disguised as a free society. He had good relations with the former Soviet Union early in his adult life and admired the idea behind their society. But he later became very critical of the former Soviet Union as it spearheaded prisoner-of-war camps, invaded Budapest, and behaved with the kind of dictatorial will that he decried in Europe. He discusses this disagreement with the Soviets in his essay "Le Fantôme de Staline" in Les Temps Modernes and describes his condemnation of the French Communist Party for submitting to the dictates of Moscow.
This is an important factor in assessing Sartre's politics: He was not a Communist. Rather, he began as a believer in humanity's historical materialism (during the period of The Flies), then he moved toward Marxism, and ultimately he ascribed to what is best termed neo-Marxism. He advocated permanent progress whereby man would correct his mistakes whenever they occurred. This is one of the reasons why he criticized the French Communists: He claimed that they acted in "bad faith," adhered to policies in which they did not believe, expressed a lack of honesty, used tricks and opportunism, and lacked critical perception in all their dealings with the membership.
Sartre's Marxist thinking began with a deep hatred for bourgeois values. He insisted that the bourgeois always ended in thinking about the self, selfishly, instead of thinking responsibly about individual contributions to the group, to society.
But if Sartre rejected capitalism on the one hand and communism on the other, he found himself happily devoted to the tenets of Marxist socialism. His concept of freedom ("be free") is not at all the same as the "Fais ce que vouldras" ("do as you wish") of Rabelais' Abbey of Thélème, in Gargantua and Pantagruel, but rather a freedom based on responsibility toward society and, naturally, toward one's own growing essence. This devotion to society at large is where Sartre comes closest to Marx's thinking.
There are, however, differences between Sartre's "system" of existentialism and Marxist politics. The differences are clearest in the early writings of Sartre: Whereas Marxism is primarily interested in the biological and social condition of humanity (with consciousness seen as a "superstructure"), Sartre focused originally on the individual, on his innermost thoughts about freedom and anguish, on the concept of responsibility and consciousness. The Marxists looked at the social group; Sartre narrowed in on the individual member of that group. Marxism was external to consciousness; Sartre placed consciousness at the very center. Marxism delineated the characteristics of human collectivity and class structure whereas Sartre elaborated a theory anchored in human experience and in individual choice.
The critic René Marill-Albérès explains the differences between Sartre's thinking and Marxism, as well as their eventual coming-together: "In contrast to Marxism, which has as its starting point cosmic, biological, and social elements, Sartre starts from human experience, from consciousness, from the individual. . . . The problem is to reconcile Marxism, which explains the individual in terms of his social conditions, and Sartre's philosophy, which cannot avoid giving first place to what is actually experienced by the individual. From Marxism, Sartre borrows the notion of the dialectic — that is, the development of a reality through several stages and through several forms, each more complex than the one that preceded it. The problem of reconciliation confronting Sartre is therefore what he calls 'totalization,' or passing from the individual to the group, from consciousness to history. . . . To resolve the issue, Sartre transports 'dialectical movement' from the collectivity to the individual and, in contrast to Marxism, sees in consciousness the source of the collectivity; it is the individual that experiences social realities, reacts, develops dialectically, and creates the social dialectic." This brings us right back to the essence of Sartrean existentialism, showing how Sartre's philosophical and political ideas were intimately woven into a coherent system of thinking.
Because Sartre prefers to examine the individual, instead of the group, his Marxism is actually a neo-Marxism. He does not dispute the claims of Marx about the social collectivity, but he insists that the individual must not be overlooked in the process. In this manner, he adapts Marxism to his own thinking, but he still remains committed to leftist politics. Both the individual and the group appear in his plays, and if you keep in mind Sartre's belief in the individual as a responsible contributor to the group, you will see in what way he integrates Marxism into his system.