Forget super PACs, their much-hyped cousins, which can take unlimited contributions but must name their donors. More money[5] is being spent on TV advertising in the presidential race by social welfare nonprofits, known as 501(c)(4)s for their section of the tax code, than by any other type of independent group.

As of Aug. 8, they had spent more than $71 million on ads mentioning a candidate for president, according to estimates by Kantar Media’s Campaign Media Analysis Group, or CMAG [6]. Super PACs have spent an estimated $56 million.

Congress created the legal framework for 501(c)(4) nonprofits nearly a century ago. To receive the tax exemption, groups were supposed to be “operated exclusively for the promotion of social welfare.” The IRS later opened the door to some forms of political activity by interpreting the statute to mean groups had to be “primarily” [7] engaged in enhancing social welfare. But neither the tax code nor regulators set out how this would be measured.

In recent years, Democrats and Republicans alike have seized on that seemingly innocuous wording to create the darkest corner of American political fundraising.      ProPublica