"In this environment, we're not taking anything for granted," the spokeswoman said. "Especially when you have a Republican candidate who is untrustworthy and trying to hide his Tea Party positions."

News of the DCCC ad buy was first reported by National Journal.

As an independent expenditure, the DCCC ads can't be coordinated with the campaign. But Bonamici on Wednesday unveiled a new ad that will start airing on broadcast and cable television the same day as the DCCC ad.

“We have to get spending under control, and we also have to get Washington’s priorities straight," Bonamici, a former state senator, says in the ad. "By ending the Bush tax cuts for millionaires, so we can protect Social Security and Medicare."

Watch the ad: