A group of Midwest Senators is urging President Trump to oppose capping Renewable Identification Numbers, or RIN prices, as part of the Renewable Fuel Standard.

Joni Ernst and Chuck Grassley of Iowa, along with Deb Fischer of Nebraska, John Thune of South Dakota, and Missouri’s Roy Blunt signed a letter of opposition to RIN caps. The letter states that a waiver cap for RIN prices would prohibit President Trump from ‘honoring his commitment’ to a 15 billion gallon RFS.

The group says a waiver cap is designed to abruptly drive down the price of RINs by reducing the amount of biofuel produced. The lawmakers suggest an alternative to reducing RIN prices, in E15 sales. The letter says that by allowing year-round sales nationwide of E15, ethanol production would increase, and lower RIN prices.

The lawmakers are seeking a meeting with President Trump to discuss “the harm a RIN waiver cap would impose” on U.S. agriculture.