OSSEO, Wis. — From the start, President Trump’s combative trade strategy toward China has carried the promise that short-term pain inflicted on farmers would be worth the longer-term gains for American producers, from agriculture to technology.

As the trade war intensifies, faith in that proposition among the president’s most stalwart rural fans is being tested more than ever.

“How long is short-term?” Shane Goplin, a sixth-generation farmer growing soybeans and corn in west-central Wisconsin, wondered as he maneuvered his 16-row John Deere planter.

China was the largest buyer of American soybeans until tit-for-tat tariffs all but halted the flow. And this month, souring prospects for a trade deal again sent prices tumbling. Mr. Trump responded on Thursday by announcing a $16 billion package to help those hurt by the dispute.