AS this historic Democratic primary season enters its next grueling phase, the party has become embroiled in a conflict between antagonists who would seem better cast as allies. Senator Barack Obama is a black candidate who has built his career on de-emphasizing race, while Senator Hillary Clinton is a white liberal who has been sensitive to minorities, and the issues facing them, during her long years of political activism.

And yet, in contest after contest, particularly in large states with diverse ethnic populations, support for the two candidates has reflected the sort of splits that normally divide Democrats from Republicans. And the pattern is likely to be repeated on Tuesday, when voters go to the polls in Indiana, a predominantly white state and North Carolina, which has a substantial black population.

How did this happen in the party that identifies itself with racial reconciliation?

The answer may begin in the identities, and ambitions, of the two candidates, but it reaches back to the tangled roots of modern political history.