WASHINGTON—An 11th-hour fight is under way over the Renewable Fuel Standard, a federal mandate created at the height of fears over high oil prices that dictates ever-increasing volumes of biofuels be blended into U.S. fuel supplies.

At issue: The U.S. is using less gasoline than expected, thanks to years of sluggish economic growth and vehicles offering greater fuel economy. As a result, policies mandating higher use of biofuels made chiefly from corn are creating headaches for refiners, auto makers and the federal government.

The Environmental Protection Agency is expected soon to release biofuel volumes for 2014, the minimum amounts that refiners must include in their fuel mixes. Most observers expect the agency will hew closely to a draft EPA proposal leaked last month, which suggested the minimum use of corn-based ethanol be cut to just over 13 billion gallons next year from this year's 13.8 billion.

The law originally called for the EPA to mandate 14.4 billion gallons be used in 2014. The cut would represent the first reduction since a 2005 law mandating biofuel use went into effect.

Biofuel producers and farm-state boosters of biofuels are undertaking a last-ditch effort to change the EPA's mind. "It makes no sense to roll back the most successful energy policy this country has had in the last 40 years," said Tom Buis, chief executive of Growth Energy, a trade association of ethanol producers and supporters. He and other proponents stress what they say are the economic and environmental advantages of ethanol.

Refiners, the oil industry and trade groups representing interests ranging from poultry producers to recreational boaters are urging the agency to stick to its reduction plan. Critics of the renewable-fuel law say, among other things, that the mandate raises the price of animal feed and represents an enormous subsidy for corn farmers, because it provides a floor for corn prices.

Ultimately, the critics want Congress to repeal the Renewable Fuel Standard altogether. They say the recent rise in domestic energy production, coupled with lower demand for fuel, has dismantled the rationale that drove Congress to establish the standard in laws passed in 2005 and 2007.

"It's just an upside-down world, compared to the assumptions" used in 2005, said Charles Drevna, president of American Fuel & Petrochemical Manufacturers, a trade group that includes refiners.

Auto makers and others also say that making gasoline blends with more than 10% ethanol could damage older vehicle engines. Using the currently mandated ethanol volumes for next year would require refiners to make blends containing more than the 10% ethanol typical of today.

Biofuel producers counter that higher ethanol mixtures are safe and boost consumer choice by providing a variety of blends.

Studies on the effects of higher blends have reached different conclusions. One, commissioned by the auto industry, suggested that higher ethanol levels could harm older engines. A recent survey of studies, carried out by the National Renewable Energy Laboratory, found little evidence for that view.

The reduced biofuel volumes envisioned in the EPA's draft for next year would keep the ethanol mix below 10%, suggesting the EPA takes the "blend wall" problem seriously.

Meanwhile, companies that use other materials to make biofuel also want the EPA to keep its current mandate, since the EPA's draft would cut requirements for these fuels as well.

Clean Energy Renewable Fuels, a subsidiary of Clean Energy Fuels Corp.CLNE -1.76% of Newport Beach, Calif., makes bio-methane from landfills, which can be turned into natural gas for transportation. The EPA's proposed tweaks "will be a sledgehammer" to his industry, said Harrison Clay, president of Clean Energy Renewable Fuels.

In Washington, the lobbying flurry has included a push for ethanol by Terry Branstad, the Republican governor of corn-growing Iowa, who has talked in recent weeks with officials at the White House, the EPA and the Agriculture Department. The governor's office said in a statement he was "optimistic that reason will prevail within the EPA" and the mandates would keep rising.

On the other side, nearly 170 lawmakers from both parties sent a letter in late October to EPA Administrator Gina McCarthy, urging her to cut back biofuel requirements.

Write to Keith Johnson at keith.johnson@wsj.com