An unprecedented leak of the EPA's controversial proposal weeks ahead of its official release further inflamed the debate.
The 2007 law mandated a total of 18.15 billion gallons of renewable fuel blending next year. The EPA's proposal requires just 15.21 billion gallons.
The EPA has warned that the country is approaching a point where the RFS would require the use of more ethanol than can be blended into gasoline at the 10 percent level that dominates the U.S. fueling infrastructure.
Refiners have said this so-called "blend wall," if left in place, would force them to export more fuel or produce less gasoline, leading to shortages and higher prices at the pump.
The use of a higher, 15 percent ethanol blend, known as E-15, is another part of the debate. The EPA has declared E-15 safe for cars, SUVs and light trucks built from 2001 forward, now the majority of the U.S. fleet. Refiners say the blend risks damage to car engines, as well as chainsaws, boats and other equipment. E-15 is in limited use in the United States.
An influential automotive group warned that auto manufacturers' warranties could be invalidated by the use of E-15. "It will likely take another decade before the bulk of the (vehicle) fleet will be able to use, be E-15 compatible, as approved by manufacturers," said Avery Ash, director of federal relations for the American Automobile Association.
The hearing room was peppered with people wearing blue T-shirts that read "Save my Engine." Energy Citizens, a group funded by the American Petroleum Institute (API), handed out the shirts to supporters and held a rally to highlight their opposition to ethanol.
The EPA proposed cutting the corn ethanol portion of the 2014 mandate from the 14.4 billion gallons called for by law to about 13 billion. Based on projected gasoline demand, that level of ethanol use would be slightly less than 10 percent of total U.S. gasoline consumption.
Thursday's meeting will not yield any immediate response from the EPA. A public comment period for its proposals will run through late January, and a final rule could take months after that.
Corn Prices at Risk
Ethanol supporters and the biodiesel industry have warned that the lower mandate could seriously hurt U.S. corn prices by undercutting demand from refiners, and trigger job losses across rural America.
Opponents of rising renewable fuels usage make related claims, pointing to upward pressure on food prices that squeeze livestock producers and chain restaurants, among others.