If you're talking about what happened around 1912 or around 1932 feel free to speculate. But how things are now or what happened in the 1860s isn't going to give you the best explanation of how things were in the 1910s or 1930s or why what happened back then happened.
The big break came when Teddy Kennedy got his pretty much 'open boarders' for the 3rd world Immigration Reform Act of 1965.
Look where those 3rd worlder's settled from the 60s on and you can pretty much tell what will be Red and what will be blue. It took most of a generation to get there, but there we are and on places like New York and California, there is no going back.
It's only going to get worse. Buy stock in the Free Obama Phone company.