The Citizens United decision — which extended free speech rights to corporations, nonprofits and unions — effectively curbed efforts to rein in political spending, while paving the way for follow-up rulings from courts and the Federal Election Commission that would unleash additional billions of dollars in donations. Those donors would spawn a boom in tax-exempt “charitable” and “social welfare” groups as vehicles for spending on political activity.

“It shouldn’t surprise anyone that some of the people attacking the IRS and supporting cuts to its budget after 2010 were the same people pushing the envelope of how to move ‘dark money’ around to maximize its political effect,” Koskinen said. “The fewer auditors the IRS had, the lower the odds of being caught.”