In Washington, a historically black church is trying to attract white members to survive. Atlanta's next mayoral race is expected to feature the first competitive white candidate since the 1980s. San Francisco has lost so many African-Americans that Mayor Gavin Newsom created an "African-American Out-Migration Task Force and Advisory Committee" to help retain black residents.

"The city is experiencing growth, yet we're losing African-American families disproportionately," Mr. Newsom says. When that happens, "we lose part of our soul."