Sixteen Republican senators have come out against President Donald Trump’s plan to allow year-round sales of ethanol blended gasoline, indicating the fine line he must walk between Big Corn and Big Oil.

A bipartisan group of 20 senators, including four Democrats, have signed onto a letter imploring Trump not to lift restrictions on the sale of gasoline containing 15 percent ethanol, also known as E15. Their opposition comes as the president is widely anticipated to make the announcement on E15 sales at a campaign rally in Iowa next week.

“A one-sided approach to addressing concerns related to the Renewable Fuel Standard that favors only one industry stakeholder is misguided,” the senators wrote to Trump on Thursday. “We are concerned that doing so would do nothing to address the policies impacting refinery jobs, could hurt millions of consumers whose vehicles and equipment are not compatible with higher ethanol blended gasoline, and risk worsening air quality.”

Some of the letter’s Republican signatories included Oklahoma Sen. James Inhofe, Texas Sen. John Cornyn and Pennsylvania Sen. Pat Toomey. Many of the lawmakers come from states where the oil industry looms large.

E15 sales are currently restricted during the summer months. However, at a campaign stop in Council Bluffs, Iowa, on Oct. 9, Trump is expected to announce that he will direct his administration to lift this restriction, allowing ethanol blended gasoline to be sold year round. Such an announcement would be widely welcomed in Iowa, where corn growers in Iowa would benefit from more demand for their product.

The opposition from many GOP senators to this move exemplifies how the White House must carefully navigate between oil and ethanol interests — two big constituencies for the president, but both at odds over ethanol mandates. Oil refineries have long argued that the mandates drive up costs and lead to job losses.

While oil state lawmakers oppose lifting E15 restrictions, senators from the “Corn Belt,” such as Iowa Sens. Chuck Grassley and Joni Ernst, are much in favor of doing so. The two Iowa Republicans have been critical of former Environmental Protection Agency Administrator Scott Pruitt’s attempt in the past to roll back ethanol mandates.

The president also appears to be making the change to help boost Republicans in the state that are fighting tough re-election battles, Iowa Gov. Kim Reynolds and Iowa Rep David Young among them. In fact, Trump’s Tuesday announcement is expected to be a part of a campaign rally on behalf of Young.

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This article originally appeared in The Daily Caller