Michael Kinsley hits it on the head:

What the press seems to value is successful spin. As gaffes and the phony umbrage that follows them gradually swallow up our politics, the press has taken on a bizarre role like that of judges in a figure-skating competition, as opposed to referees in a hockey match. What counts is the artistry. Figures in the news get points for successful spin (whether true or not), for a positive image (whether deserved or not), and for avoiding gaffes (that is, for not telling the truth.)

And, the GOP being better at spin, or at least more successful at getting people to believe lies (“death panels” anyone?), gets a better shake from the Fourth Estate than Democrats.