Nearly two years ago, the political world could only marvel at the breadth of voter support for Barack Obama. The new president had won over voters once thought to have abandoned his party for good. He'd found new reservoirs of support among groups many thought were tapped out. He energized a coalition — made up of blacks, women, Latinos, young voters and large numbers of suburbanites — that some believed would keep Democrats in power for years to come. A scant 20 months later, the Obama coalition is frayed and frazzled. A majority of those who voted for Obama still...