When the Democrat party officially became the party of LSD and Abortion in the early 70’s. The landslide for Nixon in 1972 (as you can see by election maps) was the final official shift for the south.
Prior to that they were having a tough time making that switch; always trying to vote for Wallace (a former Democrat turned independent).
The Blacks started to leave thanks to Johnson and the War on poverty.
George Wallace was a democrat who ran for president in 1964, 1968, 1972, and 1976, the only time he did not run as a democrat was for the 1968 election, when he tried third party.
See post 25.