The cause of boosting ethanol use in cars has been strongly championed by Growth Energy, an ethanol trade group led by Wesley Clark, the retired Army general and 2004 Democratic presidential candidate.

Gen. Clark's group petitioned the Environmental Protection Agency last year to allow ethanol levels in gasoline blends to be as high as 15%, up from the current 10%.

Without the increase, the group said the U.S. won't be able to meet a congressional mandate requiring some 36 billion gallons of renewable fuel to be blended into the domestic fuel supply by 2022.