Not sure I can agree. I haven't studied the man enough, but my recent reading of his history books "The Greeks: a great adventure", "The Roman Republic", "The Roman Empire", "The Land of Canaan", and others don't exactly correspond with your assessment. He was also heavily involved in the Great Books project.
From these sources, I can believe the atheist part, but raving kool-aid drinking leftist doesn't ring true. Hmm. Perhaps I'll have to reconsider.
Asimov was a convinced atheist but not proselytizing about it. His politics were likely typical of his background, secular New York Jewish intellectual. I remember reading something about his being turned off by Heinlein’s libertarian/militaristic views.
I would be real curious to see what Asimov would say when science intersects politics - i.e. global warming. I remember one essay of his about the global climate - that we were lucky that we had mostly ocean at the high altitudes rather than large land masses, and that you might avert another ice age by sprinkling graphite dust on the glaciers.