It is one of the great ironies of 20th-century American history: Hubert Humphrey, the foremost proponent of civil rights among American politicians, had little contact with African Americans until age 28.

Humphrey’s distance from people who would benefit from his legislative prowess was a result of biography and history. He was born in 1911 in the tiny prairie hamlet of Wallace, South Dakota, which had no African Americans. In 1919, he moved with his family 50 miles southwest to slightly larger …