Soliciting contributions to her campaign fund, U.S. Sen. Tammy Baldwin warned her supporters about the dangers of “dark money” — cash spent on elections by groups that don’t have to disclose their donors.In an Oct. 30, 2015 email, the Wisconsin Democrat wrote that groups backed by conservatives such as Karl Rove and the Koch brothers “have spent hundreds of millions of dollars to make sure they get right-wing extremists elected into office. And it’s only getting worse.”

Source: Ten times more ‘dark money’ has been spent for 2016 elections, U.S. Sen. Tammy Baldwin says | PolitiFact Wisconsin