Democrats were the southern majority by a longshot during slavery, Democrats founded the KKK, 21 democrat senators opposed the civil rights act of 1964 (only one of those 21 later became Republican), 78% of republicans voted in favor of the civil rights act, Richard Nixon is credited with the "switch" you're talking about but in reality the south stayed 53% democrat during his election in 1968, Democrat Jimmy Carter won the south majority in 1976. In 1992, Democrat Bill Clinton won Georgia, Louisiana, Arkansas, Tennessee, Kentucky, and West Virginia. The reason the south is Republican now isn't some mad conspiracy theory, it's because they've changed. As racism in the south decreased, the number of southerners voting Republican increased. That's why black Republican Senator Tim Scott won the South Carolina majority. Southern values have changed.
https://www.dailysignal.com/2018/12/17/fact-check-more-republicans-voted-for-the-civil-rights-act-as-a-percentage-than-democrats-did/