Why has Fred Phelps from the Westboro Church always been a registered Democrat?

Most of the Republican base are democrats. It is left over from the Civil War and WWII. Basically the idea of old bigot racist used to be democrats and republicans used to be the new age look to the future party. The roles are now reversed. Many of the democrats in the south and like phelps just continue to call themselves democrats even though they vote republican.