CANDIDATES from both parties have been talking a lot about the loss of American jobs, declining wages and the skyrocketing cost of college.

But missing from the debate is the fact that there are hundreds of thousands of “middle skill” jobs in the United States that are — or soon will be — going unfilled because of a dearth of qualified workers. Employers complain that electricians, pipe fitters, advanced manufacturing machinists, brick masons and radiology technicians are scarce. More than 600,000 jobs remain open in the manufacturing sector alone. These are jobs that provide a middle-class wage without a traditional four-year college degree.

American high schools once offered top-notch vocational and apprenticeship training, preparing young people for jobs like these. But over the last 70 years, our commitment to such education has waxed and waned, reflecting the country’s ambivalence about the role of school in preparing young people for employment and the value of blue-collar work itself. Progressives have argued that technical education tracks low-income and minority youths toward second-class citizenship; hence they often advocate “college for all.”

Over the past decade or so, however, there has been a move among educators and policy makers to reinvigorate vocational education, now rebranded as career and technical education. Some schools have been extraordinarily effective; others are struggling. If we are to offer young Americans options that are readily available to their counterparts in countries like Germany, we need to figure out what makes for success.