I agree the middle class has been hurting in the USA for a long time, due to jobs moving out and automation. But that's really one way of looking at it, United State's backbone is it's technology and innovation prowess. The system which replaces old jobs with new jobs and creates more new jobs for the whole world. That's how the policy makers think, or at least they had been before trump and no country seems to be challenging that.
The numbers/levels jobs created by new technologies pales into comparison with the number of jobs being lost due to automation, industries closing and/or relocating their production overseas.
For every factory relocating and making, say, a 100 workers unemployed, many times more are affected in terms of losing jobs or reduced income, whether as direct suppliers to the manufacturing facility being closed, or whether being dependent upon supplying everyday services to the workforce that is being made redundant.
When a large manufacturing facility closes or relocates, whole towns and communities are devastated in a ripple down effect.
Another factor is that whereas as little as 50 years ago, it it wished to do so the USA could simply isolate itself from the rest of the world and still carry on being successful since it had plenty of every kind of conceivable resource needed to be self sustaining.
But now in the interconnected globalised world, the USA is as much dependent upon the rest of the world as the rest of the world is dependent upon the USA.
For all we know US government can be in the pockets of every person in the world since two of the most widely used Operating Systems in the world (Android and iOS) were developed by American companies. That is the power of the US.
No need to just 'know' - it is an actual fact. Big Uncle Sam can shut down any part of the world it wishes to at any time, and not just because of iOS and Android in one's pocket. But doing so means playing it's ace card, which will mean the long term demise of the said ace card as the world wakes up.