Presumptive GOP presidential nominee John McCain of Arizona and 23 other Republican senators have asked EPA Administrator Stephen L. Johnson to consider waiving a requirement in last year’s energy law mandating a fivefold increase in U.S. biofuel production.

In their letter Friday, the lawmakers cited growing concerns that the biofuel mandate contributes to soaring food costs.

“American families are feeling the financial strain of these food-to-fuel mandates in the grocery aisle and are growing concerned about the emerging environmental concerns of growing corn-based ethanol,” the senators wrote. “It is essential for the EPA to respond quickly to the consequences of these mandates. Congress made the mandates in the [2007 energy law] different from existing mandates to provide flexibility and to encourage innovation in advanced and cellulosic fuels. We believe today’s circumstances merit the use of this flexibility.”

The 2007 energy law required 36 billion gallons of biofuels be produced annually by 2022. Of that, no more than 15 billion gallons may come from corn-based ethanol. That was a major ramp-up from the former mandate, enacted in 2005, which required that 7.5 billion gallons be produced annually by 2012.

Last year, ethanol production totaled 6.5 billion gallons, all of it from corn.

Critics of the higher mandate say increased production of ethanol is driving up the price of corn and tightening the supply available for animal feed or human consumption. Supporters note that it’s unlikely to be contributing to high food prices, since it mostly has not been implemented.