The US media coverage of the 2012 presidential campaign has been more negative than positive, with the harshest comments coming in social media, a study said Friday.

The 's Project for Excellence in Journalism found that coverage of President Barack Obama was not as negative as that of Republican , but that the tables were turned after the first presidential debate.

Overall from August 27 through October 21, 19 percent of stories about Obama studied in a cross section of were "favorable in tone" while 30 percent were unfavorable and 51 percent mixed.

For Romney, 15 percent of the stories studied were favorable, 38 percent unfavorable and 47 percent.

Pew said most of the advantage in coverage for Obama came in September in the form of highly negative coverage for Romney, when the was losing ground in the polls. Romney was also criticized for comments about Libya and a video in which he effectively dismissed 47 percent of the American public.

But that changed after the first debate on October 3. Since then 20 percent of stories about Romney were favorable and 30 percent unfavorable. For Obama, 13 percent of coverage was positive and 36 percent negative.

A similar study in 2008 found that candidate got mostly favorable coverage (36 to 29 percent) while articles about Republican were mostly negative (57 to 14 percent).

The Pew study said the mainstream media was not as harsh as social media, such as Twitter, Facebook and blogs.

On Twitter, the study found 45 percent of comments about Obama negative and 25 percent positive. For Romney, 58 percent were negative and just 16 percent positive.

Some 53 percent of Facebook comments about Obama were negative, as were 62 percent of comments on Romney, the study said. On blogs, Obama comments were 44 percent negative as were 46 percent for Romney.

The study also suggests the partisan reputation of cable channels MSNBC and appeared justified.

On MSNBC, just three percent of the segments studied about Romney were positive in nature, and 71 percent negative. On Fox, six percent of the segments about Obama were positive while 46 percent were negative.

Overall, in the eight weeks studied, Obama was a significant newsmaker in 69 percent of the campaign stories examined compared with 61 percent for Romney, Pew said.