It’s likely also due to the populations living in southern states, another big part of the population in southern states are those who had jobs in the mining industries or people retiring, the biggest things the republicans are pushing are bringing back mining and making sure that people get to keep their money(such as lower taxes) where as democrats are pushing for a cleaner environment(so miners blame them for losing their jobs), and major infrastructure plans that could take a while to pan out(so people retired see that and don’t want higher taxes as they already got their grain and don’t want to pass it on).

This is an over generalization and there is other major factors but these two groups are significant sections that the republicans are appealing to where as democrats aren’t such. Democrats might be able to get big wins if they could campaign on programs to help mine works get new jobs and revitalize the economies in mine towns and maybe some more programs for people that have retired so they feel they are getting more then what they’re putting in.