Sens. Elizabeth Warren (D-Mass.) and Sheldon Whitehouse (D-R.I.) urged Treasury Secretary Janet Yellen to crack down on dark money spending in political campaigns. In a letter to Yellen (pdf), the two senators argued for the Treasury Department and the Internal Revenue Service (IRS) to impose curbs on campaign spending by nonprofit organizations formed as 501(c)(4) groups, which under current laws are not obliged to disclose the source of their contributions. “The IRS’s regulation and enforcement related to 501(c)(4) organizations has been woefully inadequate in the post-Citizens United era,” they wrote. “We urge you to undertake a careful review of what the IRS has done, reform its approach, and rein in abuse by ‘dark money’ organizations.” In 2010, the U.S. Supreme Court ruled in the Citizens United v. the Federal Election Commission case, with the controversial 5-4 split decision reversing longstanding campaign finance restrictions that effectively enabled corporations and other outside groups …