The Renewable Fuel Standard, otherwise known as the ethanol mandate, requires refiners to blend an increasing amount of biofuels into the U.S. gasoline supply each year.

Created in 2005, the standard was meant to help reduce carbon emissions as well as U.S. dependence on foreign oil. But critics question how much it has helped on either score. Some say corn ethanol, the fuel most commonly blended with gasoline under the standard, actually worsens pollution. Others say the domestic oil boom has done far more to wean the nation...