We have grown up in a society in which the middle class is so much a part of the economic structure of the nation that it is difficult to imagine how the country would function without it - or that there was ever a time when there was no middle class. However, the middle class arose in large measure with the beginnings of industrialization and the shift in urbanization. While there have always been some people who might have been designated as middle class since the founding of the first city, the presence of a political important middle class did not arise in the United States until the middle of the 18th century as cities became larger and more complex and an increasing number of individuals found their calling as merchants and artisans rather than as farmers.
Agriculture as a profession tends to suppress the formation of a middle class, which tends to be defined by the portability of its skills. Farmers are tied to the land and usually tied to traditional systems of peonage (although they may not be called this). We can see this distinction between a two-class system in agrarian societies and a three-class system (with a middle class along with an upper and lower class) in an urbanizing society if we compare the Southern states to those of the North, the latter of which by the middle of the 18th century had a clear middle class while the former lacked one:
The negroes spot at the bottom of the social ladder was fixed by law. The social ladder in Carolina was the home to an immature aristocracy, for it had no real middle class. By the beginning of the eighteenth century it had created two carolinas; an aristocratic save owning and an affluent plantation society (http://members.cox.net/herodotus/writing/8teen.html).
This binary structure of the South would not change substantially during the 18th century or arguably even during the decades of the 19th century before the Civil War.
But while the rise of a middle class in t...