The film Lincoln (which I assume is true to life, at least in broad strokes) clearly shows the Republicans as the progressive party (albeit divided) and the Democrats as reactionary (pro-slavery, pro-South). But now - or in 1996 - we understand the Democrats (albeit divided) as progressive and the Republicans as conservative (reactionary).

It's as if there was a complete swap at some point. I don't know enough US history to dig through this, in short: when? And I appreciate there's likely to be a lot of sub-stories to this, like the Southern Democrats, so the more detail the better!