I think a lot of that has to do with what this very forum focuses on: Evolution and Creationism. There are many Republicans who I know personally who believe in Evolution, but if you're a Creationist then chances are you are NOT a Democrat.
I don't think that science in of itself pro-Democrat or anti-Republican, but the fact that the loudest anti-Evolution and anti-Environmental voices tend to come from the Republicans probably pushed more scientists to being Democrats.
An acquaintance of mine was an avid Tea-Party supporter (I'm personally a liberal, mostly) and went to many of their rallies. However, I remember talking with him about environmental issues and he was clearly upset about the Tea-Party's complete lack of legitimate stance on the issue. In modern U.S. politics environmentalism is becoming less of a contest between Left and Right and is degenerating into a contest between Right and Wrong.
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