John McCain has been portrayed in a “substantially negative” light over the past six weeks by the national media, according to a new study released Wednesday by the Project for Excellence in Journalism.

Barack Obama’s media coverage has been more balanced, with 36 percent of stories positive, 35 percent neutral, and 29 percent negative.

In contrast, 57 percent of stories about John McCain were negative, 29 percent were neutral and 14 percent were positive.

The negative McCain coverage isn’t coming from an inherent media bias however. Instead, the report concluded, more negative stories were written about McCain Between Sept. 8 and Oct. 16 because he was behind in the polls.

The PEJ said …

In the end, the trajectory of McCain’s coverage during this heart of the general election phase reflects the convergence of three closely related factors—the exploding economic crisis, some missteps in handling it, and the polls showing Obama moving solidly ahead. Once McCain began to fall behind in the polls, his efforts to change that were viewed from a political perspective. And on this ground, he lost footing.

The media love covering the horse race aspects of the campaign—53 percent of all campaign stories in the six-week window focused on horse-race elements, like strategy and polling.

McCain enjoyed a bounce in coverage in the week following the Republican convention, when 37 percent of stories about him were positive and 32 percent were negative. In contrast, 41 percent of stories on Obama were negative during the same time period, and only 20 percent were positive.

But that positive press changed the following week, when McCain gave his infamous “fundamentals of our economy are strong” speech. Every week after Sept. 15, positive press coverage decreased, as McCain struggled with how to respond to the economic meltdown.

Coverage of Obama has changed dramatically since the early days of campaign, when it was overwhelmingly positive. Between Jan. 1 and March 31, 2007, 47 percent of Obama stories were positive, while only 16 percent were negative, a PEJ study found. If reporters had the Obama infatuation bug early last year, they’ve gotten over it by now.

The problem for candidates is how the press works as an echo chamber. The PEJ writes in its report, “The event is covered. The effect is measured. And the reaction to that measurement by the campaigns is then examined and covered. That pattern becomes a snowball, and the trajectory of any one campaign event is magnified.”

An upcoming New York Times Magazine story on John McCain’s campaign (posted online Wednesday) shows how it has frequently tried to change the candidate’s message. These changes get amplified by the media, growing from ripples to waves. McCain and his strategists may be trying to change the narrative of his campaign, but this ends up working against them.

There are plenty of things wrong with media that only focus on the horse race. But this election, projecting a “steadiness of temperament” has certainly paid dividends for Barack Obama.