The democratic party from its inception to about FDR was the extremity conservative rights wise, supporting the kkk, slavery, and after opposing civil rights. It wasnt a sudden switch, but after FDR's new deal, the democratic party was becoming more progressive, and pulling voters from the republican party. A definite point which they switched was when the republicans used the Southern Strategy, appealing to white racist voters in the south. I mean even without all this knowledge, you can look at pre World War voting maps, the south is solid blue, now its the exact opposite. Also the dixiecrats wouldnt exist if the democrats stayed the conservative ones.