That's what he calls himself, his political views are hard to define. He's for privatizing Social Security to a point, then he's a big member of PETA, a critic of Christianity, and he supports the death penalty and abortion - he's on the left with a libertarian bent AFAIC, mainly because he's a pothead that wants to legalize zee weed and prostitution.
Of course, that's just a guess that he's a left-leaning liberal that won't vote that way in a national election. Maher has said he voted Clinton in '92, then Dole in '96. then Ralph Nader in 2000, Kerry in '04, Obama in '08, there seems to be no method in the madness.