As the New Deal began to liberalize Democrats as a whole, Southern Democrats largely stayed as conservative as they had always been, with some even breaking off to form farther right-wing splinters like the Dixiecrats. After the Civil Rights Movement successfully challenged the Jim Crow laws and other forms of institutionalized racism, and after the Democrats as a whole came to symbolize the mainstream left of the United States, the form, if not the content, of Southern Democratic politics began to change. At that point, most Southern Democrats defected to the Republican Party, and helped accelerate the latter's transformation into a more conservative organization.
After World War II, during the civil rights movement, Democrats in the South initially still voted loyally with their party. The signing of the Civil Rights Act of 1964, however, was the last straw for many Southern Democrats, who began voting against Democratic incumbents for GOP candidates.
