It just seems like another one of those myths about race that the Democrats have pushed so much that we've all started to believe it. Everybody believing it is a real phenomenon certainly serves the agenda of the Democratic Party, especially when it comes to motivating African-Americans (especially in Los Angeles, Chicago, Detroit, Baltimore and the northeast) to vote reliably Democrat with no consideration of an alternative.
If "Nixon's Southern Strategy" was so effective, how come Carter won all but one state in the South back in '76, a few short years after Nixon's "master plan"? And how come the South was the most competitive region in 1980?
Ford and Reagan both won white Southerners; Carter won by winning blacks overwhelmingly and doing reasonably well with whites.