Well nazis were one form of fascism. Italians another. Japan too.
Now we are getting ours.
If the left manages to gain more support the Petite bourgeoisie always move to the authoritarian/fascist side. When there's a serious threat to the 'socially liberal fiscally conservative' types they go to where there interest are aligned. They will side with the authoritarian ultranationalist.
They like the idea of an idea but, when you get down to it, it's self interest at its core and their material interests aren't that much different from what fascist want. At least it's not threatened in any material way.
When Weimar Germany was collapsing and the socialists rose in popularity the Petite bourgeoisie threw in with the national socialist.
You saw the same recently in Brazil.

We've already got economic fascism calling the shots in this country with both political parties serving its interests very handsomely while doing very little for the citizenry.
If socialist ideas make more headway with the public watch the reaction from the Libs.
They'll go fascist as they did in Brazil. They always do. They will here too if pressed.

If Sanders enters the 2020 race and mounts an even greater challenge to the Democratic Party I could easily see a split within the party with the centrists siding with the hard right from a real socialist challenge as just one scenario.

Maybe off the rails speculating but hey.... Trump...