Very interesting paper. Two problems that jumped out at me are: 1) income is not COL-adjusted, and 2) more importantly, there is not a term in the regression for urbanization/population density.
It is well-known that urban areas lean Democratic, and richer states are more urban. Thus the phenomenon may be reflecting urbanization rather than state wealth.
This depends on your state's economy. "In poor states, rich people are much more likely than poor people to vote for the Republican presidential candidate, but in rich states (such as Connecticut), income has a very low correlation with vote preference" [1].
[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1010426