It’s also that areas that it’s easier to have kids lean more republican. The denser a place is, the more liberal, but also less kids people have due to cost/difficulty. Opposite for less density. Also some cultural values are prolly in there. Ever seen how anti kid Reddit generally is? It’s also very left leaning. I’m very left leaning and even in surprised how anti kid the general consensus is here.

So I think it’s a correlation thing (republican places/attitudes generate more kids) than a cause (kids make you republican). I say this as a liberal new father. I want all the social safety net shit I wanted before twice as hard now. But who knows, that’s why correlation and causation are so hard to pick apart.

This article goes into it a bit. https://finance.yahoo.com/news/republicans-more-kids-democrats-lot-183722934.html