Thus far in this campaign, the only media stories that have appeared to hurt Obama are those in which his character is called into question, especially via his association with people: Jeremiah Wright, Lewis Farrakhan, Tony Rezko or indeed his allegedly angry and anti-American wife. That's a pattern. When someone doesn't have a record, you look to the company he keeps for a heuristical guide to his character (which in turn becomes a crude predictor for presidential performance.)
In a two-man race for the presidency, all comparisons are relative, and the contrast with McCain could not be starker. Remember the alleged affair John McCain was said to have had with a female lobbyist? If not, that's my point exactly. People know John McCain, and he would never.
The McCain campaign knows that their guy is a well-known war hero. No one questions his Christian faith, and no one challenges his (or his wife's) loyalty to America. Many Americans, expecially the ones the campaign is targeting, do not know much more about Barack Obama other than he is young, black, and a good speaker. There is no more fertile ground for political gamesmanship than under conditions of uncertainty or ignorance.
So let it be said that even in this election year of firsts and change, novelty can also be a liability, and it is precisely novelty and more precisely, a novice, who will invite the politics of old. Because Obama's character study has not yet been written, s/he who sets it in ink will establish the historical record for years to come.