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WASHINGTON — The Trump administration said Friday that it has lifted a summertime ban on the use of E15, a gasoline blend made of 15 percent ethanol. The move is designed to help corn and soybean farmers harmed by President Trump’s decision to impose tariffs.

Ethanol, made from corn and other crops, has been mixed into some types of gasoline for years as a way to reduce reliance on oil, among other things. However, burning ethanol-blended fuel in the summertime heat has a side effect of increasing smog — and for that reason the lifting of the ban raised objections from environmentalists. Oil companies also criticized the ending of the summertime ban because a wider use of ethanol will cut into their sales of gasoline.

Nevertheless, the change has the potential to reap political benefits for Mr. Trump as he gives the agriculture industry a policy change it has long sought. The formal lifting of the ban follows through on a promise Mr. Trump made to farmers on a trip to Iowa last fall, as he sought to shore up support for Republicans in the Midwest. The ban has been in place since 2011 and Mr. Trump has criticized it as “ridiculous.”

“I appreciate President Trump’s steadfast support for our patriotic farmers and for his commitment to expand the sale of E15 and unleash the full potential of American innovation and ingenuity as we continue to demonstrate our rightful place as the world’s leader in agricultural and energy production,” the Agriculture Secretary Sonny Perdue said in a statement.