When Barack Obama, whose mother was white, identifies himself as black, and when Bill Richardson, whose father was white, identifies himself as Hispanic, who is white?

The U.S. Census Bureau says the country will be majority-minority in 2050 -- that is, the combined number of blacks, Asians, American Indians and Hispanics will put whites in the minority. Texas and California are already there.

But the definition of white keeps shifting. Groups have been welcomed in or booted out; people opt out, sue to get in or change their minds and jump back and forth.

The deepest racial divide, between blacks and nonblacks, endures. But there also are identity shifts among African-Americans, as Sen. Obama's success suggests. Some make it into the middle class, where education and social mobility may help shape their identities as much as race does. Others are left behind in increasingly segregated schools and neighborhoods.

The U.S. has never found it easy to assign race, although it certainly has tried. A century ago, the people who did the counting -- demographers, sociologists, policy thinkers -- divided whites into three strata. They considered Nordic whites, from England, Scandinavia and Germany, the most ethnically desirable and elite, followed by the Alpine whites, from eastern and central Europe, and finally the Mediterraneans. Everyone else was identified as black, red, yellow or brown, which included South Asians.