I always saw Superman as sort of an old-school Midwestern type liberal. Sort of equal justice for all, rule of law, fair shake for the little guy, pro-American Dream etc. I don't think Superman would be sitting around on his hands, refusing to help the US liberate "Qurac" because of some vague concept of "international law."
Funny, I always saw him as a moderate to liberal Republican. Which is, sort of the same thing, I guess.
Batman OTOH is a definite conservative in my book.