Thanks to some tax-return digging, ProPublica found this week that the Karl Rove-connected Crossroads GPS actually spent at least $11 million more on political activities last year than they told the IRS. ProPublica’s Kim Barker reported:

New tax documents, made public last Tuesday, indicate that at least $11.2 million of the grant money given to the group Americans for Tax Reform was spent on political activities expressly advocating for or against candidates. This means Crossroads spent at least $85.7 million on political activities in 2012, not the $74.5 million reported to the Internal Revenue Service.

But what’s an extra $11 million spent on political activities, right? Wrong. Tax-exempt 501(c)(4) social welfare groups are limited in the amount of political spending they can do while maintaining their exempt status. And these developments about Crossroads GPS only underscore the need for more robust government oversight of political spending.

Unfortunately, this is an effort that has been made much more difficult in the wake of recent Supreme Court rulings. As Michael Keegan noted in May, the 2010 Citizens United v. FEC decision opened the door to an explosion of spending by c(4) groups like Crossroads GPS because it allowed them to run political ads as long as they weren’t using the majority of their money for electoral work.

Moreover, dark money groups sometimes attempt to underreport the political spending that they do undertake, which has not been helped by the IRS’s past reluctance to issue “bright lines” around what must be counted as political spending.

But that may change soon. The Treasury Department and the IRS are expected to issue guidance today specifying what “candidate-related political activity” entails and how much of it 501(c)(4) social welfare groups are allowed to do.