According to Pew Research, the number of Americans who are identified as being in the middle class has fallen from 62% of the population in 1971, to only 43% in 2015.
Normally, this would be a sign that the country is getting poorer. However, over the same period, the percentage of those in the lower class fell as well; albeit by only a meager 1%. On the other hand, the upper class grew from 29% to a whopping 49% by 2015. Thus, the middle class decline is more a result of them moving towards the upper class, and not the other way around.
However, when it comes to America's economic strength, the real issue is growth in incomes among the various classes from 1971 to 2015. As Pew further reported, the middle class income gains were just 34% against an upper class that saw its income rise by 47%. Thus, not only is the size of the upper income population increasing, but their share of the nation's wealth is also growing at a faster rate. The real lag in wages was in the lower class, which saw incomes only rise by 28%.
Of course, this plays into the Democrat's claim about income inequality and the rich getting richer and the poor getting poorer. But, understand, this trend is over decades with both Republicans and Democrats to blame.
The American Middle Class Is Losing Ground: http://www.pewsocialtrends.org/2015/12/09/the-american-middle-class-is-losing-ground/