Such arguments appear to have been no match, however, for the sway held by corn farmers over President Trump, who promised Iowa farmers during the presidential primaries that he would retain the mandate. The Midwestern agricultural belt is an important part of his base.
The Renewable Fuel Standard, enacted in 2005 and significantly expanded two years later, requires that refiners blend an increasing amount of biofuel into gasoline nationwide. The program grew out of an effort by lawmakers to reduce the country’s reliance on oil, prop up its struggling corn and soy farmers and, in theory, rein in rising greenhouse gas emissions.
The mandate became a headache for some refiners, who struggled under a rule that forced them to buy credits to prove they had blended the ethanol and gasoline. So when Mr. Trump appointed the billionaire Carl C. Icahn, a majority investor in a Texas oil refiner, as special adviser on regulatory matters, expectations rose for a rule change.
Mr. Icahn quickly got to work behind the scenes, pressing the issue with Scott Pruitt, who would become administrator of the Environmental Protection Agency, as well as with Mr. Trump. The E.P.A. soon issued a request for public comment on possible changes to the mandate, including a reduction in biofuel quotas.
(Mr. Icahn later stepped down from his post after scrutiny from members of Congress about whether he was influencing regulations on ethanol to benefit his financial investments. He has now been subpoenaed by the Department of Justice over his efforts to overhaul the program.)
Those moves incensed the biofuels industry and their allies in Congress. Senator Charles E. Grassley, Republican of Iowa, accused the Trump administration of a “bait and switch” scheme. In October, 38 senators urged Mr. Pruitt to back off from any changes. Gov. Kim Reynolds of Iowa called the president to plead her case.
“We commend the E.P.A. for laying to rest a year of attempts from a small group of oil refiners who have been using every trick in the book to change the established rules,” Emily Skor, chief executive of the ethanol industry group Growth Energy, said in a statement. “We are grateful to our allies in Congress and to Administrator Pruitt for working with us.”
Some recent research has shown that the predicted environmental benefits of biofuels are elusive, partly because a class of more advanced biofuels has not grown as expected.
Instead, most growth has been in corn ethanol and soy biodiesel, which has driven cultivation of both crops to record highs. Almost 40 percent of domestic corn, and almost 30 percent of domestic soy, now goes toward ethanol, according to Department of Agriculture data. Biofuels currently make up close to 10 percent of the gasoline used in the United States.
But that expansion has pushed crop production into grasslands and other previously uncultivated land, hurting biodiversity and reducing the land’s ability to store carbon, scientists warn. Bioenergy can also increase competition for land with food crops and livestock grazing.
A 2015 study by researchers at the University of Wisconsin-Madison found that between 2008 and 2012, more than 7 million acres of grassland and other uncultivated areas in the United States were converted to crop production, an area larger than the state of Massachusetts.
Moreover, some scientists have cast doubt on the very assumption underpinning the use of biofuels: that they do not raise levels of earth-warming gases in the atmosphere because the carbon dioxide the plants absorb when they grow offsets the carbon dioxide that is released when plant fuels are burned. A 2010 E.P.A. assessment of the program’s impacts estimated that the program would achieve 138 million metric tons in greenhouse gas emissions reductions by 2022.
A study published last year by researchers at the University of Michigan Energy Institute concluded that to date, biofuels had increased, rather than decreased, the country’s carbon dioxide emissions. The biofuels industry has disputed the study.
“The promise of better environmental benefits has completely failed,” said John M. DeCicco, a research professor at the institute who led the study and a former fellow at the Environmental Defense Fund. “There’s no doubt in my mind that the expansion we’ve seen so far has increased our greenhouse gas emissions.”
These fears differ from the oil industry’s complaints. Still, some environmental groups that had hoped that fossil fuel interests would have enough clout to convince the Trump administration to overhaul the mandate.
Instead, environmentalists are facing yet another defeat under Mr. Trump.
“We really thought Trump was going to do the right thing for the wrong reasons,” said Rose Garr, campaign director at Mighty Earth, an environmental group affiliated with the former Democratic Congressman Henry A. Waxman.
“Unfortunately, he didn’t even do that.”