Marx uses the term “middle class”. In the Victorian period this term was used to refer to the bourgeoisie or capitalist class. In modern Marxist terminology the words “middle class” would be replaced by “bourgeois” and “bourgeoisie“ as appropriate.
Class is defined by the position in which you stand with regard to the means of production. In capitalist society there are two basic classes: those who own and control the means of production and those who own no productive resources apart from their ability to work. The job you do, the status it might have, the pay you receive and how you chose to spent it, are irrelevant as long as you are dependent on working in order to live. This means we are living in a two-class society of capitalists and workers.
The existence of a “middle class” is one of the greatest myths of the twentieth century. In the last century, the term was used by the up-and-coming industrial section of the capitalist class in Britain to describe themselves; they were the class between the landed aristocracy (who at that time dominated political power) and the working class. However, the middle class of industrial capitalists replaced the landed aristocracy as the ruling class and the two classes merged into the capitalist class we know today. In other words, the 19th century middle class became part of the upper class and disappeared as a “middle” class. The term, however, lived on and came to be applied to civil servants, teachers and other such white-collar workers.
Having to work for an employer was how Marx defined the working class. Commodities express the amount of labor time embodied in them and that is how Marx has defined money.
The traditional division between “working class” and “middle class” implies that there is a conflict between these two groups, with the middle class being better paid, educated and housed, often at the expense of the working class. In order for the left liberal politics to maintain its appeal, the enemy had to be found, not in the abstract workings of a social system, but in the concrete everyday realities. The owning class is too remote to be tangible, and certainly too remote to be vulnerable. So the left reformers dragoon the “middle-class” into the role. Their immediate enemy is the “middle class” ie the lower /middle echelons of management, civil servants, social workers, teachers and all the other functionaries of capital. Making a supposed middle-class into an enemy is as divisive as anything dreamed up by the owning class.