Lauren Berns was browsing Talking Points Memo when he saw an ad with President Obama’s face. “Stop the Reckless Spending,” the ad read, and in smaller print, Paid for by Crossroads GPS. Berns was surprised. Why was Crossroads GPS, a group that powerful Republican strategist Karl Rove helped found, advertising on a liberal-leaning political website? Looking closely at the ad, Berns saw a small blue triangle in the upper-left hand corner. He knew what that meant: this ad wasn’t being shown to every person who read that page. It was being targeted to him in particular. Tax-exempt groups like Crossroads GPS have become among the biggest players in this year’s election. They’re often called “dark money” groups, because they can raise accept unlimited amounts of money and never have to disclose their donors.