Actually the primary reason for the South drifting away from the Democrats is that the Old Confederacy was basically conservative, and the Democratic Party was basically liberal. The Civil War and the New Deal obscured this reality (as did the conservative Dixiecrat wing of the Democratic Party) but Civil Rights pretty much finished off the weakening Democratic hold on the South.
Therefore Humphrey was certainly not assured of the South, especially given his own liberalness.
What you mean is that the Democratic party moved to the liberal wing, while the South stayed conservative.