In the colloquial sense middle class is just an income bracket usually small business owners and skilled workers. Its a fairly meaningless categorization for describing someone's relationship to society.

Small business owners (aka petite bourgeoisie) however, are the base of fascism as they are both heavily and precariously invested in capitalism. When capitalism enters a period of crisis, its the petite bourgeoisie that get politically motivated to "save" it by more fully subjugating the working class, but also intervening in the market to preserve sectors more loyal to the new fascist regime.