In reviewing the maps of the Democratic primary results, in Dave Leip's electoral atlas, I was struck by the narrow geographic base of Barack Obama's candidacy. In state after state, he has carried only a few counties—though, to be sure, in many cases counties with large populations. There are exceptions, particularly in the southern states with large numbers of black voters in both urban and rural counties. But overall, the geographic analysis has pointed up to me a divide between Democratic constituencies—a divide as stark as that between blacks and Latinos or the old and the young—which has not shown...