Last month, domestic oil production exceeded oil imports for the first time in years. That has led the oil refiners to argue that requiring ethanol use to displace oil imports is no longer so important, and that the whole system of mandates should be dropped.

At the same time, there has also been a decline in the overall demand for motor fuel, and it is poised to go yet lower. Demand had been growing, year by year, and Congress expected it to continue to do so. The 2007 law set ethanol quotas that would have meant a modest percentage increase in the amount of ethanol used in the mix.

Instead, ethanol became an unexpectedly large percentage of total fuel supply, and began to push up against the barrier of 10 percent, the maximum ethanol fraction allowable in most gasoline.

Millions of cars can handle blends of up to 85 percent ethanol, “flex fuel” vehicles, but few consumers like that fuel so few filling stations sell it.

Those that do are struggling. For example, on Nov. 6, a Sunoco station at a suburban mall in Potomac, Md., about 15 miles northwest of the E.P.A. headquarters, opened an E85 pump, with a formal ribbon-cutting and speeches by Maryland officials. But the manager, Amit Sharma, said Friday that the pump was selling only 60 to 70 gallons a day. “It’s nothing,” he said. One problem, according to Mr. Sharma and others, is that many people drive the flex fuel vehicles, but do not understand that means they can accept up to 85 percent ethanol.

In another strategy to dispose of rising volumes of ethanol, the producers have pushed the E.P.A. into allowing a blend that is 15 percent ethanol rather than 10, but few gas stations are equipped to sell that, and since it is approved only for newer cars, few stations have spent the money to refit their pumps to handle it. In addition, trade associations for companies that make power garden equipment and boat engines complain that that fuel will cause breakdowns.

The E.P.A. will now take public comment, which is certain to be voluminous, as the issue is crucial not only to corn farmers, who produce most of the feedstock for ethanol, and for oil companies, whose product can be displaced by the fuel, but also to all other corn customers, including livestock producers, grocers and restaurant chains.

Ethanol, the most prominent of the renewable fuels, divides environmentalists. It may involve modestly lower carbon production, but the increased market for it may also have increased farmers’ use of fragile, marginal land. Its effect on fuel prices at the pump is disputed.