A proposal to sharply reduce the requirement for biofuels use in the Renewable Fuels Standard (RFS) would eliminate thous­ands of jobs in Iowa and other Midwestern states, drive down farm income, damage air quality and force consumers all over the country to pay more at the pump, Midwestern lawmakers, renewable fuels leaders and farmers said last week at a day-long hearing in Des Moines.

The Environmental Protection Agency’s (EPA) proposal could even trigger a return of the...