IF you look at the state level voting pattern going back well into the 1920s. With some of the upper south states beginning to vote for a Republican as early as 1920 (Tennessee voting for Harding), 1924 (Kentucky voting for Coolidge), and then in 1928 saw the "solid south" be utterly divided with Virginia, North Carolina, Tennessee, Kentucky, Texas, and Florida all going to Hoover. FDR's multiple victories were massive throughout the country and honestly represented a bit of a fluke in the voting patterns of many States he was so popular, a fact which helped carry Truman in the 1948 election. In the 1952 election the pattern of the outer south breaking for the Republican over the Democrat continue, with Eisenhower winning Virginia, Tennessee, Texas, and Florida, and making even further inroad in 1956 adding Kentucky and Louisiana to his list of southern states. In the 1960 election, which the Democrat, JFK won, you saw Virginia, Kentucky, Tennessee, and Florida all go for Nixon.The main point of contention is whether the GOP got them aboard through racism, or because the GOP's desire to restrain "big government" liberalism for business reasons meshed with the states' rights ideology of the South, even if it was for different reasons.
The 1964 election was another fluke election and this is the usual point people like to pretend was where the "southern strategy" originated as some shift, which Nixon supposedly used in the 1968 election to make inroads in the south... however, in '68 Nixon only added South Carolina onto that list of states. He even lost Texas which had been swinging towards Republicans in other elections. Nixon's next election in 1972 can't really be indicative of any major trends anymore than FDR's previous ones or Reagan's 1984 elections can be, he was so popular that he won EVERYTHING BUT MASSACHUSETTS.
But in the 1978 election you saw most of the South revert to the Democrats for the last time, with only Virginia going for the Republicans. The 1980 election is the true tidewater mark for the shift of the south to the Republicans with Reagan sweeping the south save for Carter's home state of Georgia. As mentioned earlier, the 1984 election is entirely uninformative due to well... Reagan winning everything but Minnesota, his opponent's home state.
The entire "southern strategy" story is very convenient for the Democrats as it let's them pretend the Republicans are the racist "bad guys" and they are clean, but it really doesn't align well with the actual historic voting trends. The deep south only ended up switching sides firmly by 2000, as multiple southern states went for Clinton in 92 and 96. Further the upper and outer south had begun to be in play as early as 1920, long before desegregation became a major issue, rather there was something in Republican national politics that was drawing southern voters regardless of their position on segregation (which, it should be noted, the Republican party always opposed).
There's a much stronger correlation to a different trend: growth of the middle class. As a state developed a larger and healthier middle class, the state began to trend more Republican. This is in part why the upper and outer south began the shift earlier than the deep south, as their economies recovered from the legacy of slavery and the civil war earlier as they were less dependent on the plantation economy (and agriculture) than the deep south was (the upper south developing more mining and manufacture and Texas developing their oil reserves). This enabled a more robust middle class, and thus began the shift of those states from the Democrats to the Republicans, as the Republicans throughout the 20th century can strongly be argued to be the party more focused on "middle class" issues.