In the past decade, people, companies and unions have dispensed more than $1 billion in dark money, according to the Center for Responsive Politics. The very definition of that phrase, to many critics, epitomizes the problem of shadowy political influence: Shielded by the cloak of anonymity, typically wealthy interests are permitted to pass limitless pools of cash through nonprofits to benefit candidates or political initiatives without contributing directly to campaigns. Such spending is legal because of a massive loophole. Section 501(c)(4) of the U.S. tax code allows organizations to make independent expenditures on politics while concealing their donors’ names — as long as politics isn’t the organization’s “primary activity.” The Internal Revenue Service has the daunting task of trying to determine when nonprofits in that category, known colloquially as C4s, violate that vague standard. But the IRS’ attempts to police this class of nonprofits have almost completely broken down, a ProPublica investigation reveals.