The advertising has to be visible, but who exactly paid for it does not have to be. And there are plenty of less obvious spending (e.g. paying people to go door-to-door, phone calls, etc, etc—pay people, then claim they were volunteers?).
The advertising has to be visible, but who exactly paid for it does not have to be. And there are plenty of less obvious spending (e.g. paying people to go door-to-door, phone calls, etc, etc—pay people, then claim they were volunteers?).