Sartre's position on "Marxism" changed far less than his position on Stalinism and Russian communism, especially after Russia put down the uprisings in Hungary and Czechoslovakia. As I remember, his position in Search for a Method was that any valid existentialism must found itself on the social and economic theories of Marxian socialism.

That Sartre and Camus were members of the French Resistance should indicate that Existentialism is diametrically opposed to political oppression (from any source) and the dehumanisation of man. One could reasonably argue that such diverse political thinking as Libertarianism, classical liberalism, and socialism are all compatible with existential positions.

I would assume that as one "makes her own meaning" from life one chooses the political philosophy that most suits the meaning she has created. It should not be a surprise to see that an existential thinker would endorse a particular political stance, just as it should not be surprising that the same existential thinker would shift politically throughout her life. As experience happens "meaning" changes.

I suppose, as GoshisDead has insightfully remarked, that just as an Existentialist accepts an Ethics of ambiguity, so too would he or she accept a Politics of ambiguity. Both imply a choice, and both seem to also imply an openness to change as new situations, perspectives, or facts emerge. For this reason, one doesn't find in their writings an emphasis on rules to follow, but on the responsibility of the Self to make the best choices possible.