The Alumni Factor, a book and website that ranks colleges and universities, has surveyed thousands of alumni from 177 institutions and ranked the schools based on the political views of their alumni. The institutions whose alumni are the most liberal:

Colleges with the most liberal alumni
1 Reed College Portland, OR
2 Oberlin College Oberlin, OH
3 Macalester College St. Paul, MN
4 Grinnell College Grinnell, IA
5 Bard College Annandale-on-Hudson, NY
6 Swarthmore College Swarthmore, PA
7 Haverford College Haverford, PA
8 Bryn Mawr College Bryn Mawr, PA
9 Wesleyan University Middletown, CT
10 Smith College Northampton, MA
Source: The Alumni Factor