Gelman's blog links to a paper on poll bias by Adleman and Schilling.
The key graphic is one one above. Relative to the Gallup and Rasmussen polls, Fox is consistently pro-McCain and CBS/New York Times and ABC/Washington Post are consistently pro-Obama. NBC/Wall Steet Journal are in the middle, and CNN displays a slight McCain bias.
Tuesday we'll have a better idea who's right.
"I guess this makes sense, given that these different news outlets want to make their readers happy. It still surprises me a bit–I thought all these pollsters were pros. It’s not that polling and poll adjustment are easy or automatic–a lot of subjective decisions still need to be made–but I’d think it would be possible to do this without being influenced by your political predilections or those of your audience."
Gelman's statement contains an assumption that the purpose of a media poll is accuracy. But the real purpose of media news is to attract an audience for commercial messages.
With that in mind,
It makes sense to provide messages that your audience will like better.
It makes sense for the polls not to be too stable -- an unstable polling method can provide news that "Obama is surging" one week, and that "McCain is coming back" the next.
It makes sense to provide results that the audience hasn't seen before, that are just surprising enough to allow a teaser for the late night news.