The middle class at one time was built on manufacturing jobs done by high-school educated people, but is now based on college educated service jobs. This has limited the mobility to the middle class for high-school educated people.
Add to that, for much of the middle class, consumerism over the last 45 years (since Nixon ended Bretton Woods) has been driven by consumer lending, not by wage growth. And the chickens of consumer lending came home to roost in 2008.
I would add, the first point about the shift from a manufacturing economy was totally beyond control of the individual, however, the second point about consumerism via debt was totally within the control of the individual.
But Carter is basically blaming Reagan and the Reagan tax reforms for today's problems. That is a total load of crap. It was Nixon in 1972 and then the transition from the industrial age to the information age (predicted presciently by John Nasbitt in "Megatrends" in 1982) which created the structural changes in our economy which have negatively impacted the middle class.
The bad thing is, it is only going to get worse. Futurists and economists are predicting a wave of automation which impacts skilled service work.
And it has nothing to do with Reagan, but it is something the politicians should be addressing now.
'Haven't read it, but Cable-TV and Smartphones are adding new "rent" to family's budgets. (Not to mention gas prices for commuting to actual jobs).