The Koch-backed Americans for Prosperity, which presents itself as a "grassroots organization", received 86% of its funding in 2017 from just five big donors.
This detail is contained in a recent report prepared by the organization's auditor, CapinCrouse.
As a 501(C)(4) social welfare organization, Americans for Prosperity is not required to disclose the names of its donors. It has spent hundreds of millions of dollars since its formation in 2009, including to promote opposition to the Affordable Care Act, obstruct proposed environmental protections, and promoting massive tax cuts for corporations and the super rich. It recently ran a large scale campaign promoting the confirmation of Supreme Court Justice Kavanaugh.
Photo above is by Gage Skidmore, used under Creative Commons license.