I did a post a while back arguing that Fox News was partisan rather ideological. I didn't get very far into the obvious ethical concerns associated with having a major news and entertainment conglomerate in partnership with one of our two major political parties. Fox is able to maintain this symbiotic relationship and still keep up at least a facade of independence and respectability partially because most of the mainstream press has entered into an unspoken but remarkably well-observed non-aggression pact with Fox and the Republican Party.
Writers for papers like the NYT still criticize conservatives, but only in measured and indirect ways. They won't come out and say that an emperor is naked. Instead, they come up with all sorts of ways of saying sheer and flimsy and overly revealing.
This system has worked fairly well as long as their subjects have met them halfway. Even bomb-throwers like Ted Cruz kept up at least enough pretense of seriousness that the journalists could maintain some plausible deniability.
The problem with Donald Trump is that he doesn't give journalists any cover. He isn't actually that ideologically extreme compared to the other GOP candidates on most issues (if anything, he's to the left of the field on health care, monetary policy and the Iraq war). His comments about immigrants and support of birtherism are clearly designed to appeal to racist elements in the party, but it's not like we haven't seen other racist candidates recently and the press was remarkably OK with it.
Over the past couple of decades, the press has gotten stunningly good at not noticing things they don't want to notice. You can get journalists to ignore all sorts of lies and bigotry if you just give them an out, but that's just the thing Trump refuses to do. His whole campaign up to this point has depended on being as memorable and entertaining as possible, the ultimate reality show villain in what is arguably the ultimate reality show.
There have been other naked emperors on the stage recently but they've all played it at least a little coy. Trump is basically running around, grabbing his crotch, shouting "Hey, baby, do you want a piece of this?" then skipping away singing "I'm naked, naked, naked."
The press can't ignore Trump's behavior, but if they want to maintain any credibility and consistency, they really need to stop ignoring a lot of other candidates' behavior as well.