"We're not participating in this or endorsing their action," Peterson told KFGO Radio in Fargo in July. "We made a conscious decision not to be involved."
Two years ago, North Dakotans got a preview of the advertising barrage when Berg thwarted Pomeroy's bid for a 10th term in Congress. Outside groups spent almost $3 million, while the candidates themselves spent $5.8 million.
Pomeroy believes spending by health insurers angered by his 2009 vote in favor of President Barack Obama's health care overhaul law helped seal his fate.
Berg and Heitkamp doubt whether the outside advertising is changing voters' minds.
"At the end of the day, it's a grassroots campaign that has the biggest impact on the outcome," Berg said.
Heitkamp said she didn't like any of the outside advertising, including ads supporting her own candidacy.
"I'm not convinced that this is all that effective, especially when it goes against what people think they know about the candidate," Heitkamp said. "People say, 'Well, I know her, or I know him, and I don't believe any of that.'"
Nesheim said neither candidate is waging the kind of race he wants to see.
"Their campaign, it stinks," he said. "They aren't telling the people ... what they can do in the Senate to help us. All they're doing is digging at each other. That absolutely goes against my better judgment in voting for anybody."
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