Medicare emerged as a potent campaign issue in the 2012 election, with both Barack Obama and Mitt Romney vowing to tame its spending growth while protecting seniors. But there was little talk about some of the arcane factors that drive up costs, such as billing and coding practices, and what to do about them.  Our 21-month investigation documented for the first time how some medical professionals have billed at sharply higher rates than their peers and collected billions of dollars of questionable fees as a result.