The story
The term coastal is used specifically to refer to both California at large and the Washington D.C.-New York City areas, located on the West and East Coasts respectively, where many wealthy people live due to concentrations of commerce and financial endeavors, as well as higher costs of living. The conservative and Nazi ascriptions to specifically left-leaning individuals comes from the higher rate of people who hold progressive or leftist political beliefs in major urban areas.
Etymology from Wiktionary, CC BY-SA 4.0.