"It's quite possible for there to be elements of enthusiasm for one candidate or another," said Tom Rosenstiel, director of the Washington-based think tank. "That's a failure of professionalism if it's there. But this report can't suss it out."
McCain and Obama have received an equal amount of media attention since the conventions. The project judged 57 percent of the stories about McCain as negative, with 14 percent positive. The rest were neutral.
Obama's coverage was mixed: 36 percent positive, 29 percent negative, 35 percent neutral, the study found.
"I guess it's inevitable, but it does reflect the relentless degree to which winning begets winning," Rosenstiel said. "The polls are so ubiquitous that it is difficult for them not to be the picture frame through which the press views everything."
McCain's poll numbers have been sinking. As a result, many of the stories about him are about why his poll numbers are sinking - and how whatever he says or does is an attempt to stop his poll numbers from sinking, he said.
The economic crisis, and McCain's response to it, also played poorly for him in the press. His attempt to deflect attention with attacks on Obama and his ties to 1960s radical William Ayers did even worse, the study found. During this time, news organizations also did critical fact-checks on some McCain ads, including one on Obama and sex education.
Changes in the media have also heightened the sense of piling on, Rosenstiel said. There are more polls to report on, hence more stories to say McCain is doing poorly in them. The 24-hour news networks are paying a great deal of attention to the campaign because it gets ratings, making for more repeating of stories done elsewhere, he said.
The cable networks have been showing off new iPhone-like technology that allows them to pull up maps of the country and data, and the electoral maps haven't been good for McCain. Financial cutbacks also mean less time for news organizations to do enterprise reporting, again making for more repetition.
Rosenstiel noted that Obama's coverage was negative during the week after the GOP convention, when the surging McCain had the Democrat on his heels and talking about lipstick and pigs. McCain's negative coverage closely tracks the tone of Democrat Al Gore's during the 2000 campaign, he said.
A similar thing happened during the latter stages of Hillary Clinton's campaign against Obama, said David Gregory, host of MSNBC's "Race to the White House."
"There's a lot of focus on the campaign that is in decline to explain why that is happening, and then it becomes in the eye of the beholder whether that is fair or unfair treatment," Gregory said. "The best we can do is challenge both sides substantively."
The McCain campaign did not immediately return messages seeking comment.
Sarah Palin has received three times the press attention as the Democratic vice presidential candidate, Joe Biden, the study found. Her stories were judged 39 percent negative, 33 percent mixed and 28 percent positive.
Palin's coverage started out positive but turned when reporters went to Alaska to check on her record as governor. The study found only 5 percent of the stories were about Palin's family, most of them in the days after it was revealed her daughter was pregnant.
The Project for Excellence in Journalism studied some 2,412 stories from 48 news outlets for its study, including newspapers, Web sites and broadcast and cable news. A smaller sample, 857 stories, was used to judge the tone of the coverage.