Andy Kroll for Mother Jones:

A milestone passed in late August: According to the Center for Responsive Politics, dark-money groups—nonprofits created under the 501(c)(4) and (c)(6) sections of the US tax code—had by then surpassed $50 million on elections. These groups, unlike political action committees and candidates’ campaigns, do not have to disclose their donors. So some of the key players looking to sway election results remain in the shadows. This was a new record and seven times the amount of dark money spent by the same point on House and Senate elections in 2010. And this week, dark-money spending for the 2014 cycle reached $63 million—just shy of the $69 million in dark money spent during the entire 2008 presidential election.