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?
Aerospace engineers living in Huntsville, AL are mostly Republicans, yes
Note that military-types are over-represented in aerospace compared to other fields of engineering, and all the places I mentioned as having significant out-migration to HSV are khaki country.
I think it's also worth noting that Madison County has trended somewhat more Democratic recently. I would surmise that this demographic, especially the younger end of it, is becoming swingier. There are other places further west that I think have some similar types of industries, such as Boise, Oklahoma City, and Wichita, and it's noteworthy to see the voting trends.
