There are always malcontents. But the portion of Americans sick of crony capitalism, ruthless corporate efficiency and do-more-for-less work is growing large enough to threaten advantages big companies have enjoyed for decades.

This economic frustration, of course, is propelling the insurgent presidential campaigns of Republican Donald Trump and Democrat Bernie Sanders. Trump, who seems increasingly likely to be the GOP nominee, favors steep new tariffs on imports and other protectionist measures meant to reverse key elements of globalization and bring more jobs back to America. He has also caused publicity problems for companies such as Ford (F), Nabisco (MDLZ) and Carrier (UTX), by calling out their plans to close U.S. facilities and open new ones in Mexico.

Sanders rails repeatedly against “disastrous trade deals” and “billionaires on Wall Street who destroyed this economy,” while calling for breaking up big banks, imposing sharp tax hikes on the wealthy and giving the government more control over the economy than it has had at any time since World War II. With those ideas gaining traction, rival Hillary Clinton has moved left, opposing certain trade deals she once supported.

The revolution could peter out. It’s quite possible that Clinton—an establishment centrist on many economic issues, with close ties to Wall Street—will be the next president, more or less muzzling the roar of discontent until the next election cycle. Yet even so, the forces behind this voter outrage won’t go away, and might even intensify during coming years. And another recession is inevitable at some point, which will swell the ranks of the angry dispossessed.

The attack on capitalism that’s defining the presidential election focuses on free trade, a complex topic policymakers are beginning to rethink. Economists have long thought free trade, with minimal tariffs or other restrictions on the flow of goods between countries, would make everybody better off, overall. Goods would be produced as efficiently as possible, keeping inflation low and assuring strong returns on corporate investment. There would be losers as work shifted from high-cost areas to lower-cost ones, but displaced workers would find comparable jobs in other parts of the economy, with government aid as a backstop. Employment, productivity and living standards would rise faster than in a more closed economy that discouraged trade.

Economists are now discovering, however, that the broader benefits of free trade with some countries—China, in particular—are taking far longer to develop than they should. And they may never arrive. A new study by economists David Autor of MIT, Gordon Hanson of the University of California, San Diego, and David Dorn of the University of Zurich finds that China’s rise as a manufacturing powerhouse during the last 25 years has caused a “trade shock” in many parts of the U.S. economy that still hasn’t subsided. “Employment has fallen in U.S. industries more exposed to import competition, as expected,” they write. “But offsetting employment gains in other industries have yet to materialize.” Instead of finding other rewarding opportunities in “non-trade exposed” industries, many displaced workers suffer repeated bouts of unemployment and depressed lifetime earnings. That’s when they start paying attention to Donald Trump or Bernie Sanders.

Most U.S. multinationals have shifted work to China and other low-cost counties, because they have little choice: Once competitors lower labor costs, they have to do the same thing. When they don’t, they end up like the U.S. auto industry in 2008, which had such bloated costs, compared with foreign automakers, that GM and Chrysler declared bankruptcy and Ford nearly did.

What has been missing from 25 years of mostly unfettered globalization is an adequate safety net for American workers harmed by free trade. Washington offers a benefit known as trade adjustment assistance to workers who lose their jobs because of foreign competition, but it’s so small that the paper's author calls it “effectively inconsequential.” Employers, for their part, mostly rely on the free market to absorb displaced workers. But the market hasn’t been up to the job.

While the unemployment rate today is a low 4.9%, the labor force participation rate--the portion of eligible adults either working or looking for work--has been falling for 15 years and is now at the same level as in 1977, before women joined the labor force in large numbers. The U.S. economy has more jobs than ever, but household income, adjusted for inflation, is still lower than it was in 2000. By definition, that means the typical family is earning less.