Obama takes the world tour that McCain challenged him to do. Obama is well received at his various stops, creating news stories wherever he goes, generating favorable coverage here in the US and around the globe. And so--what becomes the storyline for all this? Of course, it's "Obama's coverage in the media is unfairly biased in his favor."
Karl Rove's maxim: Attack your opponent's strength (in 2000 it was that Gore's too intellectual and a wussie; in 2004 it was that Kerry's a fraudulent soldier), so that whenever his positive message is presented, it's colored negatively by the tag-along story. From now on in the 2008 campaign, not only will the media be mindful of, and resistant to, favorable coverage of Obama; the audience will too. The positive message will carry the negative message along with it.
And you don't think Rove's a genius?