Considering the history of the US Democratic Party with regards to race, particularly in the lead-up to and during the Civil War, but also well into the 1960s during the Civil Rights era, why did it become known as the more ‘liberal’ and progressive party? And why did the Republican Party, the party of Lincoln and fighting against slavery in the 1860s, come to be seen as conservative and not especially progressive on racial issues?

I’m hugely generalising of course, and thinking of perceptions as much as realities, but as a non-US observer it baffles me that these two parties essentially seem to have ‘swapped roles’ at some point during the twentieth century.

Happy for my question to be reframed or to clarify if necessary.