THE billionaire industrialist brothers Charles and David Koch have drawn sharp criticism for their extensive giving to libertarian causes. Though some of their organizational ties are public, many are unknown, thanks to a provision in the tax code that allows the Koch brothers and other donors, on both the left and the right, to conceal the recipients of their largess, even as they get to write it off on their taxes.

Fortunately, there is a solution to this problem: require all nonprofit organizations that engage in political advocacy to reveal their donors.

True, individuals must disclose on their tax returns the details of large gifts to charitable organizations, known as 501(c)3 groups from the section of the tax code governing them. But this information is kept private by the Internal Revenue Service. While gifts given directly through foundations must be made public, the wealthy can give without leaving fingerprints by routing money through “donor-advised funds” sponsored by 501(c)3 groups — which don’t have to publicly name their donors.

And, thanks to the Supreme Court decision in the Citizens United case, things are getting even worse. That decision now allows organizations that can engage in overt partisan work, called 501(c)4 groups, to take unlimited corporate money — again, without revealing their donors.