because since the 60s the dnc was splitting with the left wing, centrists and the dixiecrats. nixon won big time in 1972 for being at the center while mcgovern was from the leftist faction that had lost power with the death of fdr. carter only got elected thanks to the legacy votes of the south states especially as he was georgia's governor, but he did it with a much worse performance at 5-8% margin.
Anarcho-Capitalist