COURTLAND, Ala. >> In this forlorn Southern town whose once-humming factories were battered in recent years by a flood of Asian imports, Rhonda Hughes, 43, is a fervent supporter of Donald Trump. Her 72-year old mother is equally passionate about Sen. Bernie Sanders.

Disenchantment with the political mainstream is no surprise. But research to be unveiled this week by four leading academic economists suggests that the damage to manufacturing jobs from a sharp acceleration in globalization since the turn of the century has contributed heavily to the nation’s bitter political divide.

Hughes avoids discussing the election with her mother, but their neighbor Benjamin Green, 83, knows just what Washington needs. “It’ll take a junkyard dog to straighten this country out,” he said.

Cross-referencing congressional voting records and district-by-district patterns of job losses and other economic trends between 2002 and 2010, the researchers found that areas hardest hit by trade shocks were much more likely to move to the far right or the far left politically.

“It’s not about incumbents changing their positions,” said David Autor, an influential scholar of labor economics and trade at the Massachusetts Institute of Technology and one of the paper’s authors. “It’s about the replacement of moderates with more ideological successors.”

“In retrospect, whether it’s Trump or Sanders, we should have seen in it coming,” Autor added. “The China shock isn’t the sole factor, but it is something of a missing link.”

In addition to Autor, the research was conducted by David Dorn of the University of Zurich; Gordon Hanson, a professor at the University of California, San Diego; and Kaveh Majlesi of Lund University in Sweden.

“Exposure to import competition is bad for centrists,” Hanson said. “We’ve known that political polarization and income inequality track each other, but that pattern is simply a correlation. We’ve now found a mechanism for how economic changes create further political divisions.”

Parker Griffith experienced the move away from the political middle firsthand.

A so-called Blue Dog Democrat who represented Courtland and the rest of Alabama’s 5th Congressional District, he switched to the Republicans in 2009 and metamorphosed into a moderate Republican. But that wasn’t enough to save his seat.

Griffith was beaten in the Republican primary in 2010 by Morris J. Brooks Jr., who has emerged as one of the most right-wing members of Congress.

“If you’re under economic stress and you can’t provide for your family, the easiest answer is to find someone to blame,” said Griffith. “Mexicans, illegal immigrants, Obama.”

Brooks has said he would consider “anything short of shooting” illegal immigrants to get them out of the country and favors imposing heavy tariffs on China to “level the playing field” and punish Beijing for what he sees as currency manipulation.

In the case of the 5th District, which includes Huntsville and its space- and defense-related industries, as well as more industrial Florence along the Tennessee River, the move has been to the right.

But Autor and his colleagues found that in districts with heavy minority representation, similar shocks can push more Democratic districts in the opposite direction. While whites hit hard by trade tend to move right, nonwhite voters move left, eroding support for moderates in both parties, the study concluded.

As the South industrialized in the second half of the 20th century, poor Alabamians who once toiled on farms were able to secure a toehold in the middle class. In the shadow of Tennessee Valley Authority dams that supplied cheap power, thousands of workers sewed jeans and T-shirts, and could earn upward of $20 an hour in heavily unionized factories.

But the collapse of the apparel industry here in the first decade of the 21st century, following China’s entry into the World Trade Organization in 2001, reversed that process.

Nearly 10,000 manufacturing jobs disappeared. At 7.4 percent, the regional unemployment rate is well below its peak of 12.8 percent in 2010, but remains far above the national average of 5 percent.

The new paper underscores a broader rethinking among economists of the costs and benefits of policies aimed at encouraging industrial competition across borders.

“There’s a deeper appreciation for the magnitude of the impact on workers who lose their jobs,” Hanson said. “But the nature of globalization changed after the end of the Cold War and it took a while for academics to catch up.”

Until the NAFTA agreement with Canada and Mexico in 1994, and especially the entry of China into the WTO, trade deals were mostly multilateral and the rise in manufacturing imports to the United States came primarily from other advanced industrial nations like Germany and Japan.

“China and the WTO represented a shock that was way larger,” Autor said. “We hadn’t seen shocks like this because we were trading with rich countries, not highly productive developing countries with enormous labor reserves.”

To understand the connection between imports from China and political polarization, the researchers focused on the fact that manufacturers tend to localize in a specific region.

“There are these concentrated pockets of hurt,” Autor said, “and we’re seeing the political consequences of that.”

Autor and Hanson emphasize that trade is only one factor among many that have contributed to a polarizing Congress (income inequality is another, as are attitudes toward immigrants). But it has been an important one, particularly over the last decade, when Chinese imports ramped up.

This trade-induced polarization has had a significant effect on the overall ideological makeup of Congress.

The authors found that voters in congressional districts hardest hit by Chinese imports tended to choose more ideologically extreme lawmakers.

Between 2002 and 2010, districts in the top 5th percentile of trade exposure, on average, experienced a 19 percent greater drop in manufacturing employment relative to districts at the other end of the spectrum. Those hard-hit districts became, on average, far more conservative: the ideological equivalent of moving from Marco Rubio to Ted Cruz.

Some very conservative members of Congress have been sympathetic to free trade arguments in the past, but Brooks, who has welcomed support from the Tea Party, doesn’t mince words about where he stands.

“We’re going to have to do whatever is necessary to ensure that a foreign country isn’t able to successfully attack and destroy significant parts of the economy,” he said. “I was in China two weeks ago and they are going to clean our plow if we don’t act.”

Autor, like most economists, is still persuaded of the long-established benefits that global trade confers on the economy as a whole. But he recognizes that angry voters have valid reasons to be frustrated.

“It’s a matter of diffuse benefits and concentrated costs, but our political system hasn’t addressed those costs,” he said.

Some staunch defenders of globalization, like Gary Clyde Hufbauer, a senior fellow at the Peter G. Peterson Institute for International Economics in Washington, also acknowledge that the federal government has failed to adequately address the needs of workers dislocated by lowered import barriers.

But the benefit of free trade is “10 times the size of the losses,” he said. “Free trade really helps working-class people in terms of lower prices for products. The benefits are skewed toward people with lower income because they spend a much larger fraction of their income on merchandise.”

Perhaps, but that’s cold comfort to people in northern Alabama where wages are stagnant and manufacturing jobs are still disappearing.

In nearby Decatur, the big Nucor steel plant is hanging on, but it is under intense pressure from Asian imports, said the company’s chief executive, John J. Ferriola. Nucor has a no-layoff policy, but pay and bonuses have been cut at the company’s five Alabama facilities.

Does the steel industry in Alabama have a future? “Tell me what’s going to happen with imports,” Ferriola said.

And in 2014, the giant International Paper mill in Courtland closed abruptly, costing more than 1,000 people their jobs.

“Thirty years ago, it was booming,” Hughes said, pointing out where Courtland’s hardware store, the pharmacy and the five-and-dime used to be. “But those days are never coming back.”

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