He seems to have undergone a similar evolution to Christopher Hitchens
In one of his recent closing monologues he talked about someone on the right suggesting he run for President and how others--mostly on the left--have complained about him being so critical of the left nowadays. And he stated that it's not he who has changed, but the left.
I believe Hitchens expressed similar sentiments during the last few years of his life--i.e. that it was the American left that was losing it's marbles while his own politics had remained largely unchanged.