I don't have a scanner at the moment so I'll just list some examples.
The very original concept of the X-Men as a race allegory (right up to giving one of the mutant leaders the last name X), the original concept of Wonder Woman (although that sometimes tool a few steps beyond liberalism and into other territory), everything vaugely political that comes out of Stan Lee's mouth, every bronze age Green Arrow story after he grew his goatee, the secret society that wanted to take over America controled by Nixon from Captain America, the original Alpha Flight and early Weapon X comics (although you need to know Canadian Politics for that to become obvious) and everything written by Alan Moore ever.
Sure there were always a few prominent right wingers like Steve Ditko and R. Crumb but the majority used to be liberal leaning.