I assume what happened is that Jonah Goldberg accidentally read a few pages of a book one time. In it, he read some random historian in 1967 saying that Roosevelt’s New Deal had a large dose of Authoritarianism in it, and that the Fascist movements in Europe also were Authoritarian. He didn’t understand it (reading comprehension is hard, ya know?) but he did recognize some of the words. He recognized ‘Hitler’ and ‘Roosevelt’. What passes for the Jonah brain went to work and determined that Hitler was a Nazi and Roosevelt was a Democrat and therefore Democrats are Nazis QED and such. Soon the pages of National Review were filled with claptrap and (which much thanks to Antonin “Fuck Your Mother, your Liberal Bastard” Scalia) jiggery-pokery about all liberals being secret Nazis.
The meme quickly spread through the right wing echo chamber until every meth addled and cheeto stained FOX “NEWS” viewer was screaming it at the top of their lungs over at Redstate and other Neo-Confederate websites.