This is the worst Republican defeat in modern history. It reminds me of when the Democrats lost the civil war.
The Democratic Party of 1860 favored low (not none but low) tariffs and a smaller national government. The GOP favors lower taxes today and a smaller national government.
The ancestors of those Republican Whites were Democrats in 1860. The ancestors of those Democrat Blacks were organized as Republicans during "Reconstruction". Deep South States have to vote a huge majority of Whites in the Republican column to overcome the solid Black voting block and it does gain that sort of majority.
The newer dynamic is the tyranny of city people over country people nationwide. The cities vote Left and the countryside votes Right.