WASHINGTON — A generation of economists trained to believe that trade had little to do with the long decline in high-paying U.S. factory jobs is changing its mind.

Their findings are likely to fuel the opposition within President Obama’s own Democratic Party to his proposed 12-nation Trans-Pacific Partnership and similar pacts lowering barriers to international commerce.

Because manufacturing employment as a share of the workforce has been dropping for more than 40 years and the same trend has affected other developed nations, including Japan, with far less liberal trade policies than the U.S., many economists had concluded that automation was the primary culprit.

But studies examining the effect of China’s entry to the World Trade Organization in late 2001 have made the case that between 1 million and more than 2 million of the 5 million American factory jobs lost since 2000 are traceable to low-cost imports.

“The ‘aha’ moment,” said Massachusetts Institute of Technology economist David Autor, “was when we traced through the industries in which China had surging exports to the local addresses of their U.S. competitors and saw the powerful correspondence between where China had surged and where U.S. manufacturing employment had collapsed.”

Democrats and their allies in organized labor argue that trade deals kill jobs. Sen. Elizabeth Warren of Massachusetts said trade accords have “let subsidized manufacturers around the globe sell here in America while good American jobs get shipped overseas.”