Hypothesis: Trump is a symptom of the conclusion of the American nation building project.

This might sound a little bit far out, but I try to explain it as best as I can. In anthropology there's the idea called ethnogenesis, the process how diverse groups form a common ethnic identity. In modern times this is often accompanied with the emergence of a nation state. At the end of this process you usually end up with with the emergence of a charismatic leader who exploits this newfound national unity to further their own power.

Take Italy for example. The country politically unified at the end of 19th century. It was a project of nationalist intellectuals who believed in an existence of a Italian ethnicity and concluded that Italy should be ethnically defined state (a nation state). But the average person in Italy still mostly identified themselves with their region (Lombard, Sicilian, etc..). It took a while for measures, such as public education and propaganda, to convince the masses that they indeed are all Italians. Once the idea took hold, the country was ripe for the takeover by an ultra-nationalistic ideology in the form of fascism. Something similar happened in France after the revolution (Napoleon) or Germany after their unification (Nazism). Often those takeovers where hyped by palingenetic dreams of a return to an imagined, glorious past. The slogan "make America great again" comes to mind. This idea then fueled a drive of imperial expansionism and the exclusion of everyone who doesn't partipate in the project.

I think something similar is happening in the USA. The conditions aren't quite the same, e.g. the state institutions are well established. But there's a change how Americans define themselves. Instead of being a country of immigrants held together by common ideals (aka civic nationalism), certain segments of the population warm up to the idea of ethno-nationalism. One can't just become an American, one has to be born as one. Trump's attempt of ending birthright citizenship clearly reflects this line of thinking. The ethnogenesis/nation building of American is complete, admittance to the club is now restricted.

I can't predict the future, but I have a feeling this doesn't end well. I fear groups that are being labeled as un-american will be targeted by the full zeal of this new style of American nationalism. This ideology demands the full subjugation (or even extermination) of the out-group in favor of the in-group. This also has geopolitical implications, the western post-war concencensus might not survive much longer, and the resulting conflicts will bring much suffering.

TLDR: The US is experimenting with ethno-nationalism and accidentally is becoming fascistic.