She argued that if federal, state and local governments continued to promote manufacturing jobs with tax breaks and credits, employers should be encouraged to pay higher starting salaries and provide good benefits. "If you are getting a tax break or a subsidy, we should make sure those jobs are good jobs," Ms. Ruckelshaus said.
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."
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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.
The auto industry in the United States, both parts makers and large auto companies like General Motors, have staged an impressive recovery since G.M. filed for bankruptcy in 2009 and required a federal bailout. Lower salaries and efficiency gains, along with less debt, have helped make Detroit more globally competitive.
At the same time, one of the most important reasons for lower pay is the increased use of temporary workers. Some manufacturers have turned to staffing agencies for hiring rather than employing workers directly on their own payroll. For the first half of 2014, these agencies supplied one out of seven workers employed by auto parts manufacturers.
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The increased use of these lower-paid workers, particularly on the assembly line, not only eats into the number of industry jobs available, but also has a ripple effect on full-time, regular workers.
Even veteran full-time auto parts workers who have managed to work their way up the assembly-line chain of command have eked out only modest gains.
When Timothy Shelly first started working at the Faurecia Automotive Seating plant in Cleveland, Miss., he was earning $8 an hour. Nearly 10 years later, with a promotion that moved him up to managing three other workers, he earns $12.72, not much more than the rise in the cost of living over the same period.
"The work is very strenuous. It'll wear you down," said Mr. Shelly, who loads 35-pound tubs of parts on pushcarts and then walks miles every day delivering them.
Mr. Shelly can supplement his paychecks with overtime, which pays 50 percent more, but, he said, "it's not easy at all to get those extra shifts." Instead, according to Mr. Shelly, his plant regularly uses a service to hire temp workers who are paid $7.50 an hour and do not receive health insurance or other benefits.
In Ohio, Mr. Eberhardt, who works a 10 p.m. to 6:30 a.m. shift, is due for a raise of 50 cents an hour, having just completed his first year in the job. "Once in a while we get a Saturday or a Sunday shift, and that helps," he said. "It's time and a half."