This indirect version of the Hitler smear goes back to the 1950s, when émigré Marxist intellectuals of the so-called Frankfurt School, many of them refugees from Hitler, wondered why the masses of their adopted country had not yet risen up to overthrow capitalism. Their answer was that many if not most of the blue collar workers in the country that had saved them were sinister brownshirts in the making, afflicted with “authoritarian personalities.”Actually, Mike Godwin recently argued that Godwin's "law" might not actually apply to Trump --- you know, if the shoe fits, or something, idiot.
Around the same time, centrists like Arthur M. Schlesinger Jr. and Peter Viereck were appalled and puzzled by the demagogic appeal of the red-baiting Senator Joe McCarthy. They couldn’t understand why everybody in America didn’t join them in rallying behind Adlai Stevenson. For these centrists and liberals, the historian Richard Hofstadter supplied an explanation in his essay “The Paranoid Style in American Politics.” More careful historians, in Hofstadter’s time and ours, have demolished his explanation of the populist movement in terms of irrational, quasi-fascist paranoia. But the phrase “the paranoid style” is endlessly recycled by lazy journalists and editorial page columnists. And the equally dubious Frankfurt School concept of the “authoritarian personality” is likewise recycled by social scientists in every election cycle. Typically the liberal academics begin by equating regular conservatism or run-of-the-mill populism with “authoritarianism” and then predictably discover—surprise!—that “authoritarianism” thus defined is found among conservatives and populists.
Of course both sides can play the Hitler smear game. In October 1964, Republican Representative William Miller compared President Lyndon Johnson’s Great Society reform to the Hitler regime. More recently, the conservative pundit Jonah Goldberg’s book Liberal Fascism, which equated the entire Progressive-Liberal tradition from Woodrow Wilson and Theodore Roosevelt to the present with Italian Fascism and German Nationalism, was a best-seller on the right.
That Obama is the new Hitler has been a frequent theme of conservative commentators and politicians during his two terms in office. A low point came when Mike Huckabee said that as a result of the multinational Iranian nuclear deal, President Obama “will take the Israelis and march them to the door of the oven.”
All of this bears out the “law” of the Internet age put forward by Mike Godwin, an American attorney and author, that "as an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1.” But long before Godwin, the German philosopher Leo Strauss—himself a Jewish refugee from Hitler—dismissed what he called the argumentum ad Hitlerum as a cheap debating trick: “A view is not refuted by the fact that it happens to have been shared by Hitler.”
In any case, more at the link.