What factors brought about the development of an American middle class in the early nineteenth century?

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.