The liberal Navigator public tracking poll flashed a major warning sign for Democrats in mid-October, reporting a 20 percentage point jump since September in the share of independent voters concerned about the economy and gas prices. That was not the only data showing a turning of the tide in the battle for Congress. Regional challenges had shown up in internal Democratic polls from deep blue sanctums in the Pacific Northwest, New York and even Rhode Island, Democratic strategists say. Crime, a major focus of Republican advertising, has also risen as a major issue in several races. Candidates say they have...