Prior to the American Civil War, the Democratic party was heavier in the South, where they clung to slavery, while the Republican party was stronger in the North and was more into Civil liberties. Now the Democrats are stronger in the North and the Republicans in the South and the Democrats are the ones for civil rights. At what did this switch take place?
There's certainly more to be said about this, but the question's been asked before here; you might be interested in the previous answers.