I agree that if you go only at the top decile, the median democrat voter could make a bit more than the median republican voter (idk really), but if you take the whole population? I remember 2016 Sanders campaign funding.
Also, i went to West Virginia, an know the people there, happen to meet the local democrat (pro blue water, proponent of the redneck movement, basically a communist compared to the average american) and saw his electorate. I mean, they do live the good life, they eat venison and a lot of homegrown vegetables, they ride horses and sell lilies (no joke), brew their (mostly) bad beer and throw day-long parties on sunday where you jam from one PM to eleven (i improved more in two weeks there than in the last five year). But their income? i'd be surprised if they aren't in the last decile (or almost there).