In the first five months of the campaign, Obama got mostly positive coverage because he was generating positive events: rising in the polls, raising huge amounts of money, and holding events with enormous crowds. By contrast, Hillary Clinton's campaign was being questioned in the press for falling short of the expectations of her coronation.
The majority of the stories about Obama (58.6%) were primarily about the "game" of the campaign: who is winning, who is drawing crowds, who is raising money. For example, fully 15% of all stories about Obama focused on his fundraising (more than twice as much as all the other candidates), and these stories were overwhelmingly positive because his fundraising was extraordinary. But voters don't pick a candidate because of the money totals.
And 24.3% of the stories about Obama dealt with personal issues, where again Obama had a strong advantage since nothing about his background is negative. Only 14.5% of the stories dealt with policy or the public record.
A major part of the positive coverage was simply descriptive journalism. Because Obama has a positive story, merely describing his background is going to count as a positive mention. On contrast to Hillary Clinton and other well-known candidates, Obama's personal background was more likely to be described in the media accounts about him.
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Most of all, these numbers don't tell the true story about the media coverage. It's the nature of the negative coverage that matters most, not the amount of it. For Clinton, the negative media stories mostly focused on her failure to wrap up the nomination and questions about her likeability. Once she appeared in the media and seemed likeable, that false impression disappeared.
By contrast, one of the biggest negatives about Obama was the accusation of inexperience. And media stories in the format of "on the one hand, on the other hand" didn't count as negative in this study, since they're equally balanced. According to the study's authors, "In order to fall into the positive or negative category, two-thirds or more of the assertions in a story had to fall clearly on one side of that line or the other." So many stories might point out a meaningless positive (Obama is popular) and balance it with a devastating negative attack (Obama is inexperienced).
Much like the earlier media tropes about Al Gore (lied about inventing the Internet) or John Kerry (coward and traitor), the idea of Obama as inexperienced was not merely unproven but the opposite of the truth. It scarcely mattered that the accusation of inexperience was untrue; the media made it true by force of repetition. You'll look in vain for any press who pointed out the fact that Obama has more years of experience as an elected public official than Hillary Clinton, John Edwards, Rudy Giuliani, Fred Thompson, or Mitt Romney. It's almost impossible to find any media reporting the fact that Obama has far more foreign policy experience than four out of the last five presidents when they were elected.
In this way, Obama was tarred by the media with a false charge of inexperience, a charge that was almost impossible to overcome. Unlike Clinton, who could overcome her negative coverage simply by being friendly, Obama had no way he could prove "experience" with a soundbite. Rather than being a media darling, Obama has seen his campaign weighted down by press accusations of inexperience.
http://www.huffingtonpost.com/john-k-wilson/the-media-coverage-of-oba_b_70820.html