Why were white Southerns bitterly and violently opposed to Black and white Republicans exercising political power?

Until around the 60s, most southerners voted democrat for two reasons.

1) It was common to "vote as you shot". Many southerners voted for the democrats because the Union in the civil war had been led by the Republican Lincoln.

2) The Democrats at the time had many more conservative members from the south, mostly because of the first reason. Nowdays, the Democrats are a much more generally liberal party, but back then there was a mixture.