To me, the more important factor in this development was the creation of a transportation infrastructure in the United States.
Before the early 1800s, there was very little infrastructure that allowed inland transportation within the US. This made it so that it was very difficult to get goods from one place to another. Therefore, the US was really split into a bunch of little markets -- people from one could not really sell their goods in another because of lack of transportation.
Because of this, most people were subsistence farmers who grew what they needed plus a bit that they could trade with people in the vicinity. Once transportation became available, trade became possible. This made it so that more people could stop farming and move to towns. They could be supplied by farm products that could be moved longer distances and they could produce goods that could be sold over longer distances.
This does not really contradict the first answer. Instead, I am saying that the increase in transportation allowed the Industrial Revolution mentioned in that answer to occur.
I would say that the emergence of the Industrial Revolution in America helped to bring about a Middle Class component in the social and economic orders. The driving force of inventiveness, harnessing of electricity, borrowing of factory system plans from England, as well as advances made to industrial progress moved America from a nation of farms to driving and thriving factories. The middle class started to emerge as many more opportunities opened up for more people. Individuals were able to use the system of capitalism as a way to start borrowing money to make more money. This helped create opportunity for social and economic advancement. Along these lines, individuals were able to start working in factories and then try to advance to higher titles in the factory system in order to make more money. The growth and proliferation of income helped to create a situation where upwardly mobile individuals were able to etch a place for themselves in the factory and in the social and economic situation of the time period.
The first and most important factor which brought about the development of an American middle class in the early nineteenth century was the Industrial Revolution. The Industrial Revolution did not only create a wealthy capitalist class but also a new moderately well to do middle class. This group of people belonged mainly to the service sector. They were the people who reaped the benefits of the industrial revolution by contribution in no small measure to the manufacturing industries like the iron and steel industry.
The Industrial Revolution spurred consumerism. The newly rich middle class were rich enough to buy the new goods being manufactured which in turn created more demand for manufactured goods.
The Industrial Revolution resulted in new cities being established. The resultant urbanization led to the development of the middle class.