Superman has been a liberal for the last 70+ years. Don't forget how he fought fascism too.
Superman is even more liberal when you compare him to Batman. Think about it. Newspaper reporter Clark Kent (damn liberal media!) vs wealthy industrialist Bruce Wayne (a self made man! except for the inheritance part -- down with the death tax!). Over the top boy-scout rescuer of damsels in distress who gets his power from yellow sunlight vs vigilante with "anything goes that doesn't kill" attitude towards crime prevention who works in the shadows of night. It's amazing they were able to stand one another in Justice League.