Must-read Matea Gold for WaPo:

The political network spearheaded by conservative billionaires Charles and David Koch has expanded into a far-reaching operation of unrivaled complexity, built around a maze of groups that cloaks its donors, according to an analysis of new tax returns and other documents.

The filings show that the network of politically active nonprofit groups backed by the Kochs and fellow donors in the 2012 elections financially outpaced other independent groups on the right and, on its own, matched the long-established national coalition of labor unions that serves as one of the biggest sources of support for Democrats….

Lloyd Hitoshi Mayer, a University of Notre Dame Law School professor who studies the tax issues of politically active nonprofits, said he has never seen a network with a similar design in the tax-exempt world.

“It is a very sophisticated and complicated structure,” said Mayer, who examined some of the groups’ tax filings. “It’s designed to make it opaque as to where the money is coming from and where the money is going. No layperson thought this up. It would only be worth it if you were spending the kind of dollars the Koch brothers are, because this was not cheap.”