Over at the Monkey Cage:

Fact: Academicians tend to be politically differentiated according to discipline, with those in the social sciences and humanities on the left, those in the natural sciences in the middle, and those in engineering and business on the right.

Argument: This is no coincidence. Students’ political attitudes are being shaped by their professors.

Counterargument: Agreed. It’s not a coincidence. But the operative force is self-selection, not socialization.

I haven't seen any surveys, but this characterization of political differentiation jives with what I've seen. Of course no discipline is homogeneous.

As for why it works out this way? My vote is largely for self-selection. The notion that students on the right or in the middle are converted to lefties by left-wing professors is frankly crazy: there is no way professors have that much power. Professors probably do more to shape the political attitudes of students who are already leaning in the direction of a professors' beliefs.