As far as the people moving there I'm not sure. I think its most people from Alabama and other metros in the south.
It actually looks like Madison County gets a lot of high-income in-migration from Southern California, Colorado, DC suburbs, and Florida. Link.
But like Chicago is to the midwest Atlanta is to the south. Any minorities or liberal-minded whites will be more geared to moving to Atlanta. Which means demographics skew more white and more conservative.
This is a function of the MSA's industry-mix. HSV's aerospace and tech industries attract a lot of White, male engineers (i.e., conservatives) while cities like Atlanta and Nashville are more dominated by industries like marketing, entertainment, and public interest/non-profit (which are going to attract more diverse pools of workers).
Are you actually going to argue that tech bros vote Republican?
Tech =\= Engineering. Most engineers I know are more conservative than programmers or data scientists.
