Presidential candidate debates have a significant impact not just on voter sentiments but on media sentiments, according to a study by the Pew Research Center’s Project for Excellence in Journalism.

The study, to be released on Monday, used computer software and with human judgment to assess the tone of news and opinion coverage of the 2012 presidential race and found that the debates “have coincided with some notable shifts in tone.” The coverage of Rick Perry, the Texas governor who entered the race relatively late, in August, became more measurably negative around the same time he was perceived to have performed poorly at a Fox News debate on Sept. 22.

Still, the Pew study found that Mr. Perry received the greatest proportion of positive coverage of any candidate May 2 through Oct. 9. The recipient of the greatest proportion of negative coverage was President Obama.

During no week in the five months studied was coverage of Mr. Obama more than 10 percent positive. On average, it was judged to be about 57 percent neutral and about 34 percent negative. “These numbers are very stark for Obama,” said Tom Rosenstiel, the director of the project. Even the week in early May that Osama bin Laden, the leader of Al Qaeda, was killed, the positive sentiments were far outnumbered by negative ones. “That was striking,” he said.

The finding was even more outstanding, he said, when compared to the general election cycle of 2008, when the Republican nominee, John McCain, garnered markedly more negative coverage than Mr. Obama.

This time around, several Republican candidates are criticizing Mr. Obama, “and that has a kind of cumulative effect,” Mr. Rosenstiel said.

The computer algorithms set up by Pew analyze text content from thousands of news sources, blogs, social media postings and, in limited cases, television transcripts. The fact that the algorithms sense that roughly half of all the source statements, even including quotes, are neutral “shows you how much straight, factual reporting there still is,” Mr. Rosenstiel said.

The study found that coverage of Mitt Romney, the constant in the Republican race, has been similarly constant — about one-quarter positive, one-quarter negative and one-half neutral. Coverage of Herman Cain, who was largely ignored by the media until recently, has turned more positive in recent weeks. “That’s because he tends to perform fairly well at these debates,” Mr. Rosenstiel said. Coverage of Michele Bachmann, meanwhile, has turned more negative recently.

The study will be repeated through the presidential election cycle to see how media coverage and online opinions shape perceptions of the race.

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