Democrats in the 1860’s were Conservative. Republicans were the wild eyed abolitionist. The two have switched since then.
From Lincoln’s first inaugural address:
In your hands, my dissatisfied fellow-countrymen, and not in mine, is the momentous issue of civil war. The Government will not assail you. You can have no conflict without being yourselves the aggressors. You have no oath registered in heaven to destroy the Government, while I shall have the most solemn one to “preserve, protect, and defend it.”
I am loath to close. We are not enemies, but friends. We must not be enemies. Though passion may have strained it must not break our bonds of affection. The mystic chords of memory, stretching from every battlefield and patriot grave to every living heart and hearthstone all over this broad land, will yet swell the chorus of the Union, when again touched, as surely they will be, by the better angels of our nature.
I take issue with that.
Before the war, Democrats wanted to extend slavery and keep Blacks subservient.
During the war, Democrats staged draft riots to sabotage the efforts of the Union.
After the war, Democrats started the KKK.
After the war, the Democrats opposed Reconstruction of the South and opposed rights for Blacks.
The Democrats supported Jim Crow laws.
The Democrats opposed the 1964 Civil Rights Act.
The Democrats supported the New Deal and the Great Society, big government programs which encouraged people to rely on government hand-outs rather than their own efforts, therefore leading multiple generations into subservient lifestyles.
The Republicans have been on the other side of this from start to finish.
Nobody really switched sides. The Democrats have always been the Bad Guys. But Bad Guys with clever PR.