Evan Coyne Maloney makes a point I've also noticed: "conservative", "Republican" and "ties to the Republican White House" are necessary qualifiers for people like Robert Novak; however, "liberal", "Democrat", and "ties to Democrats in Congress" aren't when naming George Stephanopolis or Tim Russert or Chris Matthews.
Again, it's silly to ascribe this to some purposeful, conscious decision. But we use this kind of adjectival qualifier to point out when we're talking about something out of the ordinary: male nurse; female judge or engineer; blind watchmaker.
What I think this tells us is that there is an implicit assumption, widely enough shared that most people don't even notice, that press people and commentators are "liberal" and "Democrat" unless told otherwise.
I wonder how that changes other perceptions?