Southerners have always been poor as compared to their Northern counterparts. The heavy reliance on the slave labor made it difficult for them to let go of it. Northerners relied on commerce as opposed to agriculture. They had no use for slavery so opposition for it within the North grew without much opposition.
The legacy of the slave trade which persisted more in the south than in the north meant that Southerners were more opposed to civil rights movements than Northerners were. Again, education and wealth were more in the North than the South so that played a role as well.
Republicans have always been big on God and they used him properly. For example, they drew their arguments from the Bible and the US Constitution as they fought against slavery in the 1800s and racial discrimination in the 1960s. The SDA Church was particularly militant explaining why so many of their members today are African-American.
Dems have always supported relativism i.e. you have to evaluate something based on its present circumstance. Slavery was bad, but the economy required it. Blacks were human, but races are different.
Slowly, the Northerners, educated and wealthy, started pushing the boundaries of previously unquestioned virtues. For example, they supported abortion, gay marriages, and *freedom from religion* among other things.
The Republican Party became inconvenient for the Northerners because it always had God/absolutism as a central theme. The Democratic Party became their home because relativism is its central theme.
Conversely, the Southern States, which had always valued religion, independence, and tradition, saw that the relativism of the Dem Party had gone too far. Consequently, the Republican Party started feeling like home.
More importantly, the Republican Party valued Constitutionalism. Constitutionalism means the states retain the right to deal with matters not explicitly mentioned in the Constitution e.g. gay marriages, abortion, and cross-gender bathrooms. It also means that judges have to interpret the Constitution based on the thoughts and intentions of the founders.
Today, US judges are giving political rulings as opposed to constitutional ones. Roe vs. Wade is a classic example. The recent ruling on Trump’s extreme vetting order is also political as opposed to being judicial. It is the judicial tyranny the founding fathers feared.
Southern States want a high level of freedom from the federal government. It is important to remember that the Southern States always felt looked down upon by their Northern counterparts. They detested federal interference in state matters. Remember, the fight against slavery in the 1800s and racial discrimination in the 1960s only succeeded because of federal interference.
The founders also wanted freedom from the federal government. That means Southerners might have to fight to retain this freedom in case the federal government goes too far hence their emphasis on gun rights. For example, federal agencies under Obama forced religious institutions to buy contraceptives for their employees. Catholic universities had to remove crucifixes in their campuses for them to get federal funding.
The founding fathers warned against increased government. They knew it would attack the liberties they valued. They also called upon Americans to resist it when it arises, not if it arises. The Second Amendment is there for that precise reason. The Democrats, knowing they might have to force their decisions on some people, do not want it there.
In summary, the education and wealth of the north allowed them to start on absolute principles, but they veered into relativist ideas, as they became elitists. The South started dependent on relativist ideas, but with absolutism simmering under the surface. Eventually, their dependence on relativist ideas faded and absolutism came to the surface.