Robert Paxton is a highly cited and  influential scholar of fascism. In his1998  Five Stages of Fascism, he wrote: 

“Fascism is a political practice appropriate to the mass politics of the twentieth century. Moreover, it bears a different relationship to thought than do the nineteenth-century ‘isms.’ Unlike them, fascism does not rest on formal philosophical positions with claims to universal validity. There was no ‘Fascist Manifesto,’ no founding fascist thinker.  

Particular national variants of fascism differ far more profoundly one from another in themes and symbols than do the national variants of the true ‘isms.’ [eg, Hitler and Mussolini had very different ideologies] …fascisms are radically unique in their speech and insignia. They fit badly into any system of universal intellectual principles.”  

In his 2004 book The Anatomy of Fascism , Paxton continued to define fascism as a “form of political behavior”, not as an ideology, doctrine or set of principles.  However, in a 2024 interview, Paxton did say he considered  the “core” of fascism  to be “ authoritarian nationalism, with an enthusiastic mass base.” He has waffled on whether he’d consider Trump a fascist, having long avoided the "facile and polemical use of the fascist label". He did think Trump crossed the red line in 2021 with January 6 but has since hedged on calling him a fascist in various interviews. Paxton has noted though that Trump’s personality, especially his need for adulation, lends itself to fascist political behavior.   

I am not quoting Paxton as the ultimate authority on fascism. No scholar is. Historians and political scientists (aka “experts”) differ in their definitions of fascism and opinions of Trump. However, I have noticed that definitions of fascism have morphed over time,  perhaps repurposed to boost present-day relevance and create a tighter fit with current figures or political movements. In the process, the term has lost its complex specificity (as exemplified by Nazi Germany and Mussolini) and become loose collection of attributes, each existing along a continua of concern, with no clear boundaries between the beneficial, benign, and problematic.

For example, when does a headstrong leader become an authoritarian or a mass political base become too enthusiastic? Where are the thresholds separating tolerable and intolerable?   Hard to say until it isn’t.

References:

Paxton, Robert O. The Five Stages of Fascism Journal of Modern History , Vol. 70, No. 1 (March 1998), pp. 1-23. The University of Chicago Press. https://www.jstor.org/stable/10.1086/235001  

Paxton, Robert O. The anatomy of fascism. Vintage, 2005  

What is Fascism: A Conversation with Dr. Robert Paxton Brian Gruber / Medium. Oct 17, 2024