Running a presidential campaign takes money. A lot of money. But candidates frequently find themselves in a bind because it has to be the right sort of money. And with laws allowing your opponents to scrutinize almost all of the cash coming in the door, you’re left open to criticism if you take “bad money” from “bad people.” Enter Bernie Sanders. He’s got a solution for that problem, particularly when it comes to all of that “dark money” that comes in from PACs and Super PACs. Those dollars frequently come from unnamed sources, particularly since the Supreme Court’s decision in...