The breakdown was another blow for American farmers, who just a few weeks ago thought a trade deal was imminent. They now begin another growing season with uncertainty about who will buy their crops and whether they can break even.

Though Mr. Trump pledged Friday to help farmers — part of the coalition that put him in office — there were signs of frustration. They raised questions about his negotiating tactics. They said they were worried about what would come next given an already-struggling agricultural economy.

“Many people are just torn because people want to support the president of the United States,” said Nancy Johnson, the executive director of the North Dakota Soybean Growers Association. “But it’s very stressful to be in the middle of these very challenging negotiations. Because you’re the person who can’t take hope to the banker to get his loans for operating.”