As the tall, poised black woman takes the stage in the vast indoor sports stadium, an extraordinary wave of sound runs through the 8,000-strong crowd and bounces off the rafters: a sort of deafening ululation, a wild, excited keening unlike anything I have heard at a political rally. “Good Ev-En-Ing Pittsburgh,” says Michelle Obama, with a rising cadence, and then a pause. “Are you ready?” After his victory in North Carolina, Barack Obama seems to be building an unassailable lead in the see-sawing Democratic primary race, and for months Mrs Obama has criss-crossed America as the warm-up act for her...