Over the past few months, a steady stream of journalists from mainstream-media outlets — at least eight, led by Time Washington-bureau chief Jay Carney — have abandoned journalism for positions in the Obama administration or with congressional Democrats. Fortunately for them, the difficult transition from objective reporting to ardent advocacy of a party’s agenda was made easier by the head start they got in last year’s campaign. Though there have long been concerns about liberal bias in the media, 2008 was the year the referees took off their striped shirts and donned a team’s jersey.

Some say it’s a shift forced …