They "pointed out some of the aspects of his record that people weren't aware of and that's brought the numbers down," Romney said on Fox. "By the way, there's nothing that any of these ads by any of the candidates are showing about Speaker Gingrich that President Obama wouldn't put out with his billion- dollar money that he's going to have down the road. So it's probably a good time for people to see these things to make up their mind."
Other candidates received less negative attention, CMAG data show. Twenty percent of the ads attacked Romney. About eight percent of the ads mentioning Perry were negative.
Mahe said the negativity in Iowa is only a preview of what the Republican nominee, and President Barack Obama, will face later.
"This campaign will be nothing but negative ads on all sides," Mahe said. "We know Obama will, because that's the only story he's got. Obviously, as we've seen, there's a lot things folks have to say about Obama that could be characterized as negative."