An Iowa State University political science professor says he’s “astounded” by the amount of ad money outside groups are spending to try to sway voters in Iowa’s U.S. Senate race.

Anyone who’s listened to the radio or watched TV in Iowa has been exposed to the barrage. Researchers at Wesleyan University have been analyzing this year’s crop of political ads and they found that from September 12-25, more than 11,000 ads aired on Iowa TV stations. That’s more than in any other Senate race in the country.

David Andersen, a political science professor at Iowa State University, says the campaign conversation now is being “driven” by the ads purchased by outside groups rather than by the candidates themselves.

“The candidates are losing their voice,” Andersen says. “It is these other groups that are running the campaigns now and kind of putting words in the candidates’ mouths.”

Nearly 63.5 percent of the ads run in Iowa’s Senate race from mid-to-late September were negative. Only 16 percent were positive. Negative ads aren’t a new phenomenon, but Andersen says it’s “amplified” this year by the outside spending.

“It’s very hard to get excited about voting for people you don’t know,” Andersen says. “You don’t understand them and the advertising right now is very opaque in explaining these candidates.”

Andersen studies voter behavior and the influence of social media.

“What I find really interesting is just how much of the conversation is not being driven by the candidates and their campaigns, but how much is being driven by these out-of-state groups that are trying to win an Iowa senate race because of their interests,” Andersen says. “I think a lot of the conversation is about the Koch brothers and how they’re attacking Braley and the climate change groups that are attacking Ernst — and the candidates themselves aren’t a part of this.”

However, the negative tone of the ads isn’t going to depress voter turnout that much, according to Andersen, but he warns the “lack of substance” in the ads might.

“Negative ads can really have substance when you say: ‘This candidate stands for something that you do not,’ but right now the negative ads aren’t necessarily saying that,” Andersen says. “They’re just saying this is a bad person in a very unusual way.”

Through the end of September, outside groups spent an estimated $233 million on this year’s races for the U.S. House and Senate and governors races around the country.