Tompkins County, New York

[Adam Griffin]

Probably Cornell University and Ithaca College. Combined, their enrollment makes up about 30% of the county's population (I could be wrong, but it wouldn't surprise me if that's the largest student population as a % of any county in NY).

[Adam Griffin]

Ithaca is very liberal, and it's not just students.

Pretty much this. The universities only enhance the leftward bias of the city

I'm not sure if this is what you meant by 'enhance', but it's not just in a quantitative fashion. The campuses there surely alter the flavor of leftward thinking to a degree as well (explains the Teachout/Wu/Obama examples pointed out by OP), to the point that even if the county only voted as D as the surrounding ones, it still probably would exude the same tendencies as it does now.