The Wall Street Journal has an interesting article and accompanying video about the McCain families in Carroll County, Mississippi: the descendants of John McCain's ancestors who still remain in the area where those ancestors owned a cotton plantation (Teoc), and the descendants of the slaves who worked that plantation and took their owners' surname. The McCains have a biannual reunion where family members of both groups of descendants meet.
Charles McCain, grandson of a slave at Teoc, the white McCain plantation, was active in the Civil Rights Movement:
Charles McCain was a central figure in the local chapter of the National Association for the Advancement of Colored People. When civil-rights workers swarmed Mississippi in 1964, the black McCains housed white activists and received bomb threats and harassing calls.
Link to Charles McCain record in the Mississippi Sovereignty Commission
Link to Charlie McCain
To find all McCain Links -- Mississippi Sovereignty Commission