WaPo:

The record amount of independent spending swamping this year’s congressional races is fueling a deeply negative political atmosphere, as attack ads from independent groups overwhelm the messages of candidates in many competitive races.

More than 80 percent of the money spent by big-money independent groups and parties in the general election has gone to oppose a candidate, according to a Washington Post analysis of campaign finance data collected by the Center for Responsive Politics, a nonpartisan research organization.