By itself, the state of the economy is enough to guarantee a close election, and every national survey during the past two weeks has put Obama and Romney in a statistical tie. Now another key factor points in the same direction—the shifting balance between the political parties. This matters because party preferences and voting patterns are more closely linked today than they have been in several generations—and two recent in-depth surveys of the party system document that a clean Democratic victory, of the sort the party enjoyed in 2008, is exceedingly unlikely. The surging Democratic tide of four years...