Based in New York, NELP is a research and advocacy group for low-wage and unemployed workers. It receives some financial support from organized labor, including unions like the United Steelworkers and the United Food and Commercial Workers, as well as the A.F.L.-C.I.O. The data in the study were drawn mostly from government sources like the Bureau of Labor Statistics and the Census Bureau, and independent experts confirm many of the trends NELP cites in the report.

“We are not going to back to Detroit in the 1950s or Akron in the 1900s,” said Lawrence Katz, a professor of economics at Harvard. “There still are many manufacturing jobs that are high-paying, but they tend to be more senior or require a lot more education than entry-level jobs do. And their numbers are shrinking, too.”

Even if it does not continue to be a mass employer or a ticket to the middle class, Mr. Katz said, manufacturing remains vital to the economy because it spurs innovation and leads to higher-paying, value-added jobs like design, marketing and other support services. It is also a major source of productivity gains, as well as a generator of profits and exports for American companies.

But most of those gains are not going to the workers on the factory floor. One major factor in the downward pressure on overall manufacturing wages has been a particularly sharp fall in hourly pay earned by workers like Mr. Eberhardt who make parts that supply the big automakers. Parts workers make about one-third less than assembly-line workers who put together cars and trucks, but parts jobs account for 72 percent of all auto sector employment. From 2003 to 2013, median wages for parts workers fell to $15.83 an hour from $18.35.