Sorry, but you're just stuck on stupid.
In 1860 Southern slaveholding Democrats ruled in Washington, DC, and had for nearly 60 years, since they won the election of 1800.
Those slaveholding Democrats had allied themselves with Northern Democrats representing Big City immigrants (think: Tammany Hall) and the globalist commercial classes -- just like today.
And just as today, Republicans represented very different groups of people -- rural farmers, small towns, small businesses, skilled workers, religious & constitutional conservatives, suburban professionals and the like.