Economy is indeed a direct driver. But can this be a theory of "everything"? Texas is not cheap. The liberals could look at cheaper living options as well. Rentiers in Boston, Seattle must be doing great. And what about the proportions of singles?

The article with the map says:

When Lesthaeghe studied the map county by county, he found the link between family size and political leaning became even stronger.

It then acknowledges that (basically) your economic drivers increasingly pressurre the conservatives as well. It makes a guess that the fertility predictive power will last until 2020. A probable dominant factor for liberal optimism is mentioned (birth rates rise amongst women of color).