A group of economists that includes Messrs. Hanson and Autor estimates that Chinese competition was responsible for 2.4 million jobs lost in the U.S. between 1999 and 2011. Total U.S. employment rose 2.1 million to 132.9 million in the same period.

In the 2000s, congressional districts where competition from Chinese imports was rapidly increasing became more politically polarized, the two researchers and two coauthors concluded from examining vote totals. “Ideologically strident” candidates replaced moderates, they wrote in a paper.

In this year’s Republican presidential primary races, Mr. Trump won 89 of the 100 counties most affected by competition from China, according to an analysis by The Wall Street Journal. Those counties include Hickory’s Catawba County, where Mr. Trump got 44% of the Republican vote in the March primary against 11 other candidates.

Mr. Sanders won Democratic primaries in 64 of the 100 most-exposed counties in northern and Midwestern states. That pattern didn’t hold in the South, where Hillary Clinton was strong among black voters.