“When the farmer does well, so does the rest of the United States,” said Ms. Riessen, who declined to say whom she voted for in 2016. “And we’ve just been kind of struggling.”

Mr. Trump, who has accused China and other countries of taking advantage of the United States on trade, said he would help farmers weather the trade uncertainty. On Twitter, Mr. Trump suggested that the United States government could buy billions of dollars of agricultural products from farmers. Sonny Perdue, the agriculture secretary, said separately that Mr. Trump had directed his department to work on a plan to help farmers.

“@POTUS loves his farmers and will not let them down!” Mr. Perdue said on Twitter.

In Shelby County, Ind., Phil Ramsey said he appreciated the president’s reasons for revisiting trade deals, but said the ailing farm economy had been brutal in deeply personal ways. He said he was going without health insurance to save money. He said he has delayed some equipment purchases.

“I was very patient a year ago,” said Mr. Ramsey, who grows corn, soybeans and wheat. “I’ve gone from being very patient to being very anxious.”

But Mr. Ramsey, a Republican who voted for the president, said his primary frustrations were with China, which the United States accused of reneging on some trade promises, and with Congress, which has not approved a new trade deal with Canada and Mexico.

“He can’t do it by himself,” Mr. Ramsey said of the president. “He needs the support of all the U.S. government.”

The lack of a trade deal was especially painful in Nebraska, which saw widespread damage from flooding in March. The damage there, as well as in parts of Missouri and Iowa, has turned cropland into debris fields and forced some farmers to evaluate whether they could continue making a living off the land.