Since September, Americans for Prosperity, a group financed in part by the billionaire Koch brothers, has spent an estimated $20 million on television advertising that calls out House and Senate Democrats by name for their support of the Affordable Care Act…

Campaign experts said they believe that the early advertising blitz has driven down the support for Senate incumbents in highly competitive states such as Louisiana and North Carolina that are critical to the Democratic Party’s push to hold its majority.

Some House Democrats in competitive districts find themselves under steady assault with little ability to respond unless they want to dip into money they will need later in the campaign.