From the Media Research Center:

The "National Survey of the Role of Polls in Policymaking," completed by Princeton Survey Research Associates for the Kaiser Family Foundation in collaboration with Public Perspective, a magazine published by the Roper Center for Public Opinion Research, was released in late June 2001.

The poll questioned 1,206 members of the public, 300 "policymakers" and 301 "media professionals, including reporters and editors from top newspapers, TV and radio networks, news services and news magazines." Significant findings from the survey of media professionals appear below.

KEY FINDINGS

The Kaiser Family Foundation survey found that members of the media were four times as likely to identify themselves as "liberal" than as "conservative:"

...Similarly, the survey found that members of the media were more than seven times more likely to identify themselves as "Democrat" than as "Republican"

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There were lots and lots of self-declared "moderates" and "independents" too, which means that the vast majority of people who report the news lean left. To them, it's inconceivable that there is any other way to think or be.