It's honestly not too bad. I'm a freshmen here and am quite conservative; I was expecting the liberalness to be overwhelming but it isn't fortunately. There are obviously more liberals here than conservatives but it isn't that bad.
The city of Madison as a whole is a lot more liberal than the school itself is I think. The precincts encompassing the freshmen dorms went for GOP candidates this fall at a higher rate then any other precinct in the city. There were a good number of campaigners for Ron johnson and Scott Walker around campus and a good number of people celebrating after they won.
The school used to be a lot more liberal and activst but this is dieing off quite a bit. This fact annoys the hell out of a lot of old timers who want the place to go back to "how it was". (As in back to the days when student Veitnam "activists" bombed Sterling Hall and killed someone) All the old hippies that live around town always come to campus and try to reliving the glory days by holding rallies and peace dances near State St but the majority of students are too busy, indifferent, or drunk to care. If by liberal you mean annoying hippie activists holding rallies you'll be fine. If by liberal you mean smoke weed and drink ALOT of alchohol, then you might want to rethink Madison because both activities are done in large quantities here.
The thing is, as UW becomes much more selective and harder to get into, it's becoming a lot less, atleast visably, liberal. You'll see the hipsters around but they're nowhere near the majority. Much more common now is the engineer and business major who studies his ass off all week and then gets completely wasted on Fridays and Saturdays and doesn't have time for politics.
Liberal or conservative though, everyone loves the place and the Badgers. There's no other better place to go to college for four years in the country.