From John Sides at The Monkey Cage blog:
What this shows is that every point in the past 10 months, Obama has received more negative coverage than positive coverage. The tone of Romney coverage has shifted depending on primary campaign events, but, as of the end of April, positive coverage still outweighed negative coverage. At that point, Romney received about as much positive coverage as Obama received negative coverage. … But, in the news media as a whole, the PEJ data suggest that there hasn’t been “blatant bias” toward Obama.