I'm not sure where this idea comes from originally. The two people I'm fairly certain have said it at some point are Michael Malice and Thaddeus Russell. The idea is "Politics is downstream from culture."
In the aftermath of the Unite the Right march in Charlottesville, Trump said there were probably good people on both sides and bad people on both sides, which the left interpreted as his refusal to denounce neo-nazis. This, and other comments and baseless extrapolation on their part has led the left to believe that Trump is a closet Nazi or Nazi sympathizer. We all know this isn't true but why do they think this?
The answer is not in what Trump does or says, it's the people who support him. David Duke endorsed him, and Richard Spencer led a sieg heil when he was elected. But obviously Trump didn't secretly orchestrate this, they just like him for one reason or another.
The reason the left might think he's a closet Nazi is because of the people who supported him.
They think that politics influences everything else, they think politics influences culture.
This is the central planning mindset, that everything emanates from a certain politician or political movement which inspires everything else in the culture. When they see Richard Spencer leading a sieg heil and saying "Heil Trump, heil victory," they think Trump has influenced this. This is the opposite of reality. In reality, people like Richard Spencer latch onto somebody like Trump. And because Richard Spencer just isn't very popular, we know that Trump's base is not neo-nazis, it's some other demographic.
There is a segment of the culture that already existed that liked Trump, the 5 neo-nazis in America just attached themselves onto that group.
But this central planning mindset also shows itself in the flip side. The reason they think that Trump must be a Nazi because they can see neo-nazis saluting him means that they think politics creates culture. They think this is true, which explains why they think having "their guys" in office means so much. It explains why they freak out over elections and why it's so imperative to vote.
This corresponds to their philosophy in general. They believe that changing the world is as simple as passing laws, because they think politics creates culture, when in reality it's the other way around.
If you thought that politics created culture and had the ability to create the world as you see fit, you'd also think there was much more importance in voting and the presidency. You would think that if only you had the right guy in charge, you could alleviate poverty, make everybody not racist, and change anything else about the world. This is quite obviously not how the world works, but it is the paradigm of how the average person on the left thinks.
When they say Trump is a nazi, this is not crazy. It is quite literally the only rational conclusion they can conclude based on their reality.