This transformation is no longer just about factory workers, whose ranks have declined by 5 million in the past 25 years as manufacturing moved to countries with cheaper labor. All kinds of jobs that pay in the middle range -- Clark's $17 an hour, or about $35,000 a year, was smack in the center -- are vanishing, including computer-code crunchers, produce managers, call-center operators, travel agents and office clerks.

The jobs have had one thing in common: For people with a high school diploma and perhaps a bit of college, they can be a ticket to a modest home, health insurance, decent retirement and maybe some savings for the kids' tuition. Such jobs were a big reason America's middle class flourished in the second half of the 20th century.

Now what those jobs share is vulnerability. The people who fill them have become replaceable by machines, workers overseas or temporary employees at home who lack benefits. And when they are replaced, many don't know where to turn.