In the mid-1950s LIFE magazine published a five-part series titled The Background of Segregation, exploring in depth how the politically charged issue played out, in human terms, from the post-Civil War Jim Crow South to the first fiery stirrings of the Civil Rights movement. Here — on the eve of the 2012 South Carolina GOP primary, in which the issue of race is again making headlines — LIFE.com presents rare and unpublished color photographs by the great Margaret Bourke-White. The pictures were shot in South Carolina for the third installment in the series — a remarkable article titled “The Voices of the White South” that featured white citizens from different walks of life who, for myriad reasons, fervently supported segregation. Above: An unpublished Margaret Bourke-White photograph of a black maid preparing a white family’s supper in Greenville, South Carolina, 1956, one of many photographs that was not included in the final article.