It is his ability to bridge universes that is central to his appeal  particularly to Americans under 40, a post-baby-boom generation that has intuited how interconnected the globe has become and thirsts for a new American lexicon, a new approach to a transformed world.

“We first started corresponding after my dad died,” Auma, who was born the year before her famous brother, tells me. “Barack’s handwriting is exactly the same as my dad’s. And he wrote to me on this large yellow foolscap paper that Americans use and that my dad used. It was weird.”

His words, linking estranged siblings, crossed universes. Auma’s Kenyan mother, Grace Kezia, who now lives in Britain, was pregnant with her when her father left in 1959 to study in Hawaii. There the older Obama would meet Ann Dunham, from Wichita, Kan., and their son, the Democratic presidential candidate, would be born in 1961.

Auma stayed behind with an older brother. Later, after her father’s return to Kenya in the mid-1960s with degrees from the University of Hawaii and Harvard, Grace had two more sons with him. Three other boys, half-brothers, were born to two other women, before the older Obama died in a Nairobi car crash.

Image Roger Cohen

Both of Obama’s parents perished young: his father at 46, his mother at 52. His life has been pitched forward. No wonder, as he describes in his first book, “Dreams From My Father,” he quested for a past, in which lay identity. Auma was the conduit to that.