Many of the Southern whites were not too keen on the efforts towards an egalitarian society after the end of the Civil War which was largely promoted by Republicans from the pro-Union states. Due a reactionary effort by racist white leadership, who seized control of the then moribund Democratic Party (called the Redeemers) to drive a wedge between poor whites and poor blacks succeeded by making blacks the enemy. They succeeded beyond their dreams. Segregationist laws were passed in the late 19th and early 20th century. The KKK was reborn in 1915 to intimidate blacks, keep them from voting and to make sure whites enjoyed the fruits of being at the top of the economic ladder, even if poor whites never really got there. The Redeemers, and their spiritual descendants, also did a masterful job of propaganda by promoting the "Gone with the Wind" image of the Old South as a peaceful pastoral setting beyond time and place where prosperity abounded under the the feudal leadership of wealthy white men and slaves were happy in their slavery, and genteel white women were content in their powerlessness. Blacks needed to be kept separate from whites because the African American mind could not comprehend the subtleties of white democracy--or so they said.
By the mid-20th century, segregation, which was now manifest in almost all parts of the USA to greater and lesser degrees, was ruled unconstitutional by the Supreme Court. The "South" by this time became solidly conservative Democratic with an occasional populist liberal/lefty, but social change was slow and the established institutions thwarted efforts at social reform. The rise of the New Deal offered hope to poor blacks and poor whites and reinforced the power of liberal element within the Democratic Party. One conservative segregationist Democrat, Strom Thurmond of South Carolina, left the Democratic Party to eventually join the Republicans because he so upset at the call for civil rights for blacks among the emerging progressives in the Democratic Party. The Republicans, hungry for wins in the South and beyond, began to actively court the segregationist Democrats and were able to win many over the the Republican camp with thinly veiled racist campaigns and money. With segregation now illegal, the newly embolden conservatives, like the Redeemers before them, were able to ensure that money and resources were steered towards wealthy whites while minimizing resources to non-whites. Today, in the Sunbelt, starting slowly about 25-30 years ago that ceiling started to crack as the plaster began falling and the rotted wood was exposed. The sunshine is now coming through in places. The times they are a-changin'.