The United States has always had something like a middle class, but for most of our history it has been a distinction of class not necessarily dependent on income or wealth.
In that sense we have been unusual among nations in allowing a Ben Franklin, Abe Lincoln or Harry Truman to do the same jobs as a George Washington or Theodore Roosevelt. Class mobility remained a constant feature of our republic for most of its history.
In the last century, the middle class became defined by something different: household earnings. As unpleasant as it is to say, the truth is this economic definition of the middle class was built upon an abundance of well-paying but relatively low-skilled jobs. For almost half a century, these jobs have been in retreat, while at the same time our educational attainment has stagnated.