How did the Southern United States shift from being historically Democratic States to now stronghold Republican States?

You might be interested in some threads listed in the FAQ under the section "Changing roles of Republicans and Democrats":

What changes occurred in the last century that prompted the southern states choose to switch majority support from Democrat to Republican? - 13 comments, over 2 years old.
Some short summaries and a few books for further reading.
How and when did the Republican Party transition into the Democratic Party we see today (and vice versa)? - 8 comments, over 2 years old.
The commenters in this thread focus on the 20th century changes in the Democratic and Republican parties with direct relevance to the Southern switch in loyalty.
Can someone address a brief history of Democrats vs Republicans, specifically the change in Dems from the early 1900s being against civil rights to a more progressive party in the 50/60s leading much social change in the U.S. - 66 comments, over 2 years old.
This thread mostly goes over the party changes between the 1870s and the 1960s which you might find useful as context for the above thread.
It seems that during the Civil War and reconstruction era, Republicans were liberal and Democrats were conservative. When and why did this change? - 17 comments, over 2 years old.
One of the commenters gives a very short summary about how muddled both parties were historically when analyzed through through a conservative/liberal dichotomy.