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WASHINGTON — When President Trump visits Iowa on Tuesday he will unveil a pro-ethanol perk aimed at soothing corn and soybean farmers in the heartland made anxious by his decision to impose tariffs on China, a move that kicked off a trade dispute with a major buyer of American agricultural products .

The trip to Council Bluffs, Iowa, which comes on the heels of campaign rallies in Minnesota and Kansas, is part of Mr. Trump’s efforts help Midwestern Republicans facing tight midterm election races this November because of a backlash to his trade policies. Lawmakers acknowledged that the plan — which will include lifting a federal ban on summer sales of higher ethanol blends of gasoline, something the industry has long sought — will be critical to assuaging farmers in Iowa and elsewhere who have grown deeply worried about the falling prices of corn and soybeans.

“There is anxiety about the president’s program on putting on tariffs,” said Senator Chuck Grassley, Republican of Iowa, though he noted the administration’s successful renegotiation of a trade deal between the United States, Canada and Mexico has allayed some of those worries.

Still, he said that for farmers, the ethanol announcement “would be a big boost not only because of the anxiety of tariffs but because we’re having another record corn crop and that naturally drives down prices.” Mr. Grassley has long pushed to allow the sale of higher ethanol blend gasoline in summer months. The restrictions have been in place out of concerns that burning more ethanol in hot weather contributes to smog.