Though the press (i.e., the MSM) have not been associated with one of the parties since the early 19th century, it is curious that the question of the partisanship of the press has been raised with much intensity in the last five or ten years. Attempts to measure partisanship, however, have proven hopelessly contentious and inescapably political. One can prove that the media is almost entirely liberal, or the opposite, that there is hardly a genuinely liberal sentiment to be found. For it all depends on how one defines "liberal" or "conservative" bias.
Trying to measure media bias by directly investigating media content seems to be a dead end. It is not helpful because one can demonstrate anything with this approach.
I would like to suggest that there is a better way.
If the press were significantly and systematically biased to the right or to the left, and if it is true that most people who seek out news consume news sources that reflect, rather than challenge, their political views, we would expect to see a considerable stability of political opinions. For it is true as any political science truth goes that people who are little exposed to opposing views exhibit stability of political opinions. So if the media is markedly biased one way or another, and people are watching it, their opinions are being reinforced rather than destablized, and we should be able to observe stability of political opinions.
But this is precisely what is absent in our political climate. Today more and more people change the way they vote from one election to the next, and there are many people who don't identify with any existing party. This political instability amounts to indirect evidence that the media are not as partisan as they are made out to be. Or at least that people are in fact exposed to opposing views. This is of course only a highly suggestive conjecture. My evidence is inconclusive because there may well be other factors which better account for the instability of political opinions today than the media.