NYT:

Over the last two years, government watchdog groups filed more than a dozen complaints with the Internal Revenue Service seeking inquiries into whether large nonprofit organizations like those founded by the Republican political operative Karl Rove and former Obama administration aides had violated their tax-exempt status by spending tens of millions of dollars on political advertising.

The I.R.S. never responded.

During the same period, the agency singled out dozens of Tea Party-inspired groups that had applied for I.R.S. recognition, officials acknowledged on Friday, subjecting them to rounds of detailed questioning about their political activities. None of those groups were big spenders on political advertising; most were local Tea Party organizations with shoestring budgets.