… is from page 114 of Steven Pinker’s excellent 2018 book, Enlightenment Now  (all dollars below are 2014 dollars; link added):
The study  found that in absolute terms, Americans have been moving on up. Between 1979 and 2014, the percentage of poor Americans dropped from 24 to 20, the percentage in the lower middle class dropped from 24 to 17, and the percentage in the middle class shrank from 32 to 30. Where did they go? Many ended up in the upper middle class ($100,000-$350,000), which grew from 13 to 30 percent of the population, and in the upper class, which grew from 0.1 percent to 2 percent. The middle class is being hollowed out in part because so many Americans are becoming affluent.
DBx: Note that the study to which Pinker here refers, one done by Stephen Rose, was published in 2016 by the Urban Institute – an organization not known for possessing any libertarian or conservative biases.