Slavery was the omission or suppression of freedoms and privileges that were considered rights. Slaves were bought and sold, and didn't have the right to choose to work (or not), or for whom. Slaves were not afforded the right to participate in democratic process.

Middle-class Americans are not restricted the way slaves were. A middle-class American is not restricted by law from deciding not to work, or for pursuing other career opportunities. They are allowed to vote. They are allowed to travel freely, and choose where they will live. To own property.

The thing that truly separated slaves from free people was not an income gap. It was that slaves were legally restricted from living like free people.

The concern will rise if and when middle class people are truly institutionally restricted from participating in society. If they were to institute policies to say only people with net worth over some value, or only landowners were allowed to vote. Or if laws were passed solely based on the lobby of big business to the detriment of the majority opinion. Or if people were legally restricted from changing jobs, or forced by law to work.

Some of these things are actually coming to pass. Particularly the manipulation of government and law by the very wealthy. The greater the income gap becomes, the more legislative power the wealthy have. The more legislative power the wealthy have, the easier it becomes for them to change the laws to maintain that power. When that power becomes enshrined in law, then we can start comparing the middle class to slaves.

We're not there yet, but I think there are people with a lot of wealth who would much rather see us there than lose their wealth.

This is a very bizarre analogy. The claim is that the real average wage isn't a good barometer of the economic and social condition of the middle class, despite often being held up as such. Nobody would claim that the existence or absence of slavery isn't a good barometer of the economic and social condition of the (potential) slave class.

"spending by households on many of modern life's "basics"—food at home, automobiles, clothing and footwear, household furnishings and equipment, and housing and utilities—fell from 53% of disposable income in 1950 to 44% in 1970 to 32% today."

There's a reason why the middle class today can buy so much MORE stuff, it's all much cheaper. The article is saying people are not richer, but the quality of life is not significantly different for the middle class then it is for the rich (the same access to long life, short travel, etc...)

"spending by households on many of modern life's 'basics'" The article factors in auto payments as a basic, but not gas prices - which have risen for people since the 1950s. It does not factor in the increased amount of money going to their HMO (some of which is then sent as a dividend to GMO stockholders, there's a racket...) It also does not factor in that people have to spend more on their own training and college education just to make the same wages as someone without these things could make at a factory in 1950. You would be correct in saying food prices have fallen in the past 60 years, but their claims that the middle class now has more disposable income is false.

The fact which they can not deny is that inflation-adjusted hourly wages have fallen since the early 1970s. Many of the points they make are sophistic. They do make the point that commercial airplane travel has improved since the 1970s, but how often does the average middle class person use that? Most of the people I know who fly regularly are made to do so by their jobs and are usually unhappy having to spend so much time away from home in order to make a living.

I wonder if the wall is being worn down now that RM owns it.

EDIT: People who are interested in the middle class squeeze should really check out Elizabeth Warren's book The Two-Income Trap.