Journalists do not need an ideological bias to misreport the news. Their simple preference for a good story by itself can lead to error. Just look at environmental reporting. Any nut with a placard can proclaim the End of the World, and reporters will snicker. A nut with a PhD. does the same, and it becomes Page One material.
Thankfully, at least nine out of ten apocalypses have proven wrong.
But you don't have to talk about the end of the world to hype something. . . .
There's less crime than "the well-informed" think there is, for instance. Reporters report the exceptional, but news viewers see these exceptions as the rule, with crime endemic. Thankfully, this is something that John Stossel repeatedly reminds his viewers. Few other journalists do.
Similarly, a recent study has shown that journalists, by concentrating on the biggest spending electoral races, encourage widespread misperception. In a survey conducted by social scientists at MIT and Stanford, it was found that "people with less education (and thus lower tendency to read newspapers) had, on average, the most accurate estimates of the average amount of money spent in politics and the relative importance of interest groups."
Informed readers' opinions on the subject, on the other hand, closely tracked the lopsided reporting they'd been exposed to. They over-estimated the impact of corporate and PAC money; their estimates of amounts spent on campaigns was over seven times that actually spent.
So, on average, the people in the nation with the most accurate view of politics are the least informed. At least on this issue.
Undoubedtly some reporters are so upset by politicians shilling for political investments that they deliberately over-report the top-spending races. But money can't but help be a story, as Hollywood and business reporting indicates. So simply by "going for the story," journalists (biased or not) contribute to a grossly distorted vision of modern political reality.
This is not to deny that journalists are, on average, biased to the left. I've recently argued that this is indeed the case. The evidence is in, and settled. But the extent to which the news gets skewed without conscious attempt is at least as important a problem.