Representative Rod Blum, a Republican, represents much of eastern Iowa and is facing a highly competitive race in what is the second-largest soybean-producing congressional district in the country. He and other politicians are facing a “nervous” farm community across the state, according to Grant Young, an Iowa-based Republican strategist.

“I listen to the farm show over the noon hour on WHO daily,” Mr. Young said of Iowa’s leading radio station. “They are usually a happy-go-lucky bunch promoting industry and holding a two-hour infomercial for the Farm Bureau. But the last couple of months I’m wondering if they need to take the sharp objects out of the studio.”

In Kansas, Bob Henry, who grows corn and soybeans in another up-for-grabs House district near the Nebraska border, said the country could ill afford to tangle with a market that American farmers rely on.

“For the United States soybean grower, China is the 800-pound gorilla in the room,” Mr. Henry said. He suggested that Beijing is exacting political payback against the Republican heartland: “China knows who got Trump elected.”

After an initial round of tariffs on a modest share of American exports, the Chinese have displayed a more keen awareness of the electoral map and moved to punish those industries whose misfortune will be felt most intensely in states and districts pivotal in 2018.

Karl Rove, the former strategist to President George W. Bush, said a trade clash “would limit Midwestern enthusiasm from our base and limit our ability to hold what we have and pick up more seats.” Mr. Rove also grumbled that Mr. Trump “has little to no understanding of the farm coalition.”

He may have a slightly better appreciation after a meeting last week in the West Wing with a small group of farm belt Republican senators and governors, during which two of them brought up the adverse impact that tariffs on exports could have in the midterm election, according to officials briefed on the conversation.