NYT:

The ratio of negative ads, which are defined as those in which a campaign mentions its rival by name, tells the story. Since April, after Mr. Romney became the presumptive nominee, Mr. Obama broadcast negative commercials 118,775 times compared with 56,128 times for positive commercials.

In the same time period, Mr. Romney ran negative spots 51,973 times and positive spots 11,921, according to an analysis from Kantar Media, which tracks political advertising. This does not include the Republican “super PAC” ads that are almost entirely attacks on the president.

“President Obama was keeping a more balanced mix, but by the end of June he turned off the positive spigot,” said Elizabeth Wilner, who studies advertising for the group.