It’s also important to remember that, despite Chomsky’s intense hatred of Israel, his real idee fixe has always been the United States. It’s only as Israel starts to draw closer to the US following the Six Day War, and especially after the Yom Kippur War, that he really gets going.
It’s for this reason that the question of his remaining pro-Israel really isn’t a question at all. As soon as Israel became an important ally of the United States, Chomsky could never have been pro-Israel even if he’d wanted to be. It would have thrown his entire worldview into disarray.
I would say, though, that he solidified the position of the Left on Israel and certainly gave it a lot of ammunition. He also played an important role in giving anti-Israel ideas a legitimate place in the American intellectual debate—especially in academia—and in making it a sort of litmus test for Jewish Leftists.
A lot of the things he wrote in the wake of the Six Day War were denunciations of fellow Jewish Leftists for not being “real” Leftists because of their Zionism. So as a collaborator in what was basically a purge, and in ensuring that Jewish Leftists knew that the price of their continued participation in the movement was their support for Israel, he did play an essential part.
He was also one of the anti-Israel Left’s first and probably most important shields against accusations of Anti-Semitism. Since he was one of the most prominent Jewish intellectuals in America at the time (mainly for his linguistics work), he gave the anti-Israel Left a lot of cover, and allowed them to escape responsibility for the Anti-Semitic aspects of their ideology for a long time.
It was really only with the Second Intifada that people finally started speaking out against Leftwing Anti-Semitism, which was mainly the fault of the movement itself. They’d gotten a free pass for so long that they probably thought it would go on forever. In a sense, thankfully, this has to count as one of Chomsky’s greatest failures…