TAMPA -- A possible explanation for Democratic Florida Senator Bill Nelson's recent dramatic downward slide in polls is a series of PACs that are spending freely on negative ads against him. In the space of two months, Nelson, seeking a third Senate term, has gone from 10 points ahead of conservative Republican challenger Connie Mack IV to 10 points behind, at least as measured by Rasmussen. During this time Congressman Mack, son of former Florida U.S. Senator Connie Mack III, has done little or nothing to account for the movement. (Mack faces two opponents in the Aug. 14 primary, but...