Actually, I have concluded that the causation runs in the other direction. Journalism is about promoting the image, influence, and profitability of journalism. Politicians who go along and get along with that propaganda wind emanating from journalism get positive labels, and politicians who do not do so get negative labels.Moderation is a classical virtue, promoting liberty is an American virtue, and the possibility of progress is an American belief. Thus, the politicians who cooperate with journalism get labelled "liberal," "progressive," or "moderate."
And the reason that "journalism" can be spoken of as a single entity is that wire services homogenize journalism to such an extent that there isn't a dime's worth of difference between any of the papers belonging to the AP (or any other wire service). Apart from the ghetto known as the editorial page, of course. And even there . . .