The Southern Strategy is the name of the Republican plan to use “the dividing line” ... “of the race issue” to “cut the Democratic Party and country in half,” as described by by H. R. Haldeman, Oct 5, 1971 in his Nixon White House memo Dividing the Democrats. As the memo described, white disapproval of the Civil Rights efforts by Democrats transformed the South from Democratic to Republican. Before the Southern Strategy, positions on civil rights and other social issues varied within both parties.
Ian Haney-Lopez's How the GOP became the “White Man’s Party” — From Nixon to Rand, Republicans have banked on the unerring support of Southern white men. Here's how it came to be. gives a longer, solid description of how moderate Republicans made a conscious decision to stoke white racism to gain votes.
In my opinion, the Southern Strategy is at the heart of the transformation from the Republican Party from being the party of pragmatism, hard work and small business, or of Jonathan Haidt's Moral Foundations, into the party of Gingrich and eventually Trump.