The Environmental Protection Agency on Friday officially opened up year-round sales of higher ethanol fuel blends, called E15, fulfilling a promise President Trump made to Iowa corn farmers.

Trump has firmly backed the year-round sale of 15% ethanol fuel, despite opposition by the oil industry and refiners who argue the fuel would harm vehicle engines.

The E15 rule, a boon to corn farmers, follows billions of dollars the administration has doled out to farmers as compensation for the losses they've suffered from tariffs imposed in the trade war with China.

Farmers and the ethanol industry have been calling for the reversal of EPA's Reid vapor restrictions on the fuel for years to allow E15 to be sold year-round, instead of just in the winter, spring and fall months.

Allowing E15 to be sold in the summer opens the ethanol market up to the largest driving season of the year, with farmers expecting to see demand rise for the corn-based fuel.

Most of the U.S. gasoline supply already contains 10% ethanol. The 15% blend is being sold by such large gasoline chain stores as Sheetz and KwikStop in anticipation of the rule's rollout.

Iowa Republican Sen. Chuck Grassley recently told the Washington Examiner that the ethanol rule would give Trump an edge over his Democratic rivals in the 2020 presidential election. He explained that it would be hard for his rivals to demonstrate the same commitment to farmers as Trump has in passing this new rule.

Nevertheless, Grassley and fellow Sen. Joni Ernst, R-Iowa, said that Trump's trade wars could cancel any of the gains the ethanol rule would provide for farmers.

EPA is also granting waivers to oil companies allowing them to forgo blending ethanol under the Renewable Fuel Standard program. The RFS requires refiners to blend renewable fuels in the nation's gasoline and diesel supplies. The ethanol industry argues that the refinery waivers are destroying demand for their product.

Industry lobbyists say allowing the sale of E15 year-round cannot make up for the effect of the waivers, which effectively reduce the amount of ethanol by over a billion gallons annually. Oil refiners say the ethanol industry's claims are exaggerated.