It's strange how a philosopher who was - and still is - popular in leftwing circles (as well as among “radicals”) should have enunciated a position on free will that's very similar to that which is advanced by conservatives and religious people (both in history and still today). That is Jean-Paul Sartre's belief (at least in his existentialist phase) that we have total (or complete) freedom of will.
Well, Sartre himself rejected his earlier “existentialist position” when he embraced Marxism in the 1960s. He once said that his earlier views were a “disgrace”. He did try to fuse existentialism with what he called “humanism” and Marxism; though Marxism seems to have come out on top.