There is an Obama majority in American politics, symbolized by Monday’s throng on the Mall, whose existence is both the consequence of profound changes to our nation’s composition and values and the cause of changes yet to come. That majority, as the president made clear in his remarks, would not exist but for Americans’ struggles to expand our foundational belief in the equality of all men. The drive to expand equality, he said in his speech’s most historically resonant line, “is the star that guides us still, just as it guided our forebears through Seneca Falls and Selma and Stonewall.”...