Liberalism and environmentalism have exported the heavy manufacturing industries that were used by generations of hard-working but less educated people to get a toe-hold in the middle class, so their families could live in relative comfort. They sent their children to college, and those children turned around and invented earth day, gaia, and the idea that steel mills, oil refineries, coal mines, and appliance factories were a blight on the earth.
Today, the people who would have been able to find well paid jobs in those industries are selling crack to their neighbors’ kids and voting for Democrats, who give them a pittance and re-make the economy for the benefit of college-educated dilettantes like themselves.
That's certainly a part of it. But an even bigger part of the decline of jobs in the manufacturing sector is an increase in efficiency. It takes a lot less man-hours to produce a widget now than it did a generation ago. The US's manufacturing output continues to expand, even though the actual number of jobs in manufacturing keeps decreasing, in relative terms.
Bone-headed liberal policies certainly haven't helped the manufacturing sector, but automation and other increases in efficiency have made job-losses inevitable.
It's a bitter pill to swallow, but manufacturing jobs are going away worldwide. Some countries have temporarily been able to use cheap wages to attract jobs, but eventually those wages will rise and that labor will be automated.