U.S. oil and gas companies have been an engine of growth through much of an otherwise lackluster economic expansion, providing steady employment, solid wages and fierce competition for workers across wide swaths of the country.

Now, after a roughly 50% plunge in oil prices, exploration and production companies are cutting capital budgets, service companies are weighing layoffs and nonenergy firms that popped up to support the industry are bracing for a protracted slowdown.

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