Alan Greenblatt for Governing:

Electoral politics has been all but overrun in the past couple of years by 501(c)(4)s and other vehicles that enfeeble the idea of contribution limits and free up outside groups to influence the course of campaigns with virtual anonymity. Now they are being used heavily between election years as well. Incumbent officeholders are finding that 501(c)(4)s, inelegantly named after a section of the tax code, are excellent ways to advance their personal agendas. They may not meet the definition of slush funds, but some of them come close.

Most of these nonprofit groups set up by elected officials outside the election process operate under an entirely different set of rules from those that apply during a campaign. They officially list themselves as social welfare organizations, and most 501(c)(4)s indeed used to be charities set up to advance a specific mission of public benefit — the Sierra Club promoting environmental policies, for instance.

But it turns out that the definition of social welfare can be stretched by these groups to serve purely political purposes. The only formal requirement is that political activity not make up the majority of their work. But that rule is almost never enforced. The IRS not only doesn’t ask 501(c)(4)s for the identities of their contributors, but it also isn’t curious about what the groups are up to. That’s why the media has dubbed such operations as “dark money.”