When Karl Marx wrote "The Communist Manifesto," he believed that the working class was being exploited by the "bourgeoisie." In French, "bourgeoisie" (roughly) means middle class. However, he used it in the sense of the "ownership class." But, that was more than a century and a half ago. Times have changed.
Today, the ownership class is made up of a tiny percentage of the people, perhaps as little as one-half of one percent. The modern bourgeoisie are the people who manage the business owned by the wealthy.
Marx thought that eventually the working class, which was increasingly concentrated in cities where communication was easier, would become politically aware of the fact that they were being exploited. At that point, they would be transformed into the "proletariat." A proletariat understands that they have been slaves to the owners, will rise up in rebellion, and overthrow the bourgeoisie.
As the United States becomes more and more a two-tiered society (rich v. poor), it is conceivable -- though not likely -- that the middle class could join the working class to form a proletariat.
The reason that this is unlikely is because, although the middle class is shrinking and those still in it bear the brunt of the tax burden, people with middle incomes simply have too much to lose. This is the result of having an extremely wealthy society.
But, even as times get tough for middle-income earners, most will think that the risk of upsetting the apple cart is too great. They may be the subjects of unfair exploitation, but they still have more than most people in the world. Even if their dollars become like nickles, they'll want to hold onto those five-cent pieces.
Karl Marx (with Frederick Engels), "The Communist Manifesto"
William Domhoff, "Who Rules America"