Rumors of potential action in the price of biofuel credits have Midwest politicians on edge, including a group of senators that have requested a meeting with President Donald Trump to discuss the issue.

In a letter sent to the White House on Thursday, five Senate Republicans tell Trump they are “opposed to applying a ‘waiver cap’ mechanism of any kind” to the Renewable Fuel Standard. Concerned about proposals to put a cap on the price of Renewable Identification Numbers, the credits used to measure RFS compliance, the senators say “the consequences of a waiver would be severe and immediate across the Midwest, impacting farmers and biofuel stakeholders alike.”

“We feel it is very important to let you know our strong opposition to placing a waiver cap on RINs that is intentionally designed to undermine our shared commitment of 15 billion gallons of annual biofuels production,” the senators write. “We therefore request a meeting with you at your earliest convenience to discuss the harm a RIN waiver cap would impose on the American agriculture sector, as well as offer constructive solutions that represent the win-win solutions you are seeking.”

The letter’s signatories include Missouri’s Roy Blunt, Nebraska’s Deb Fischer, South Dakota’s John Thune, and Chuck Grassley and Joni Ernst of Iowa.

Grassley also took to Twitter on Thursday night to signal his concern to Trump that a RIN price cap would lead ethanol to the “doldrums.”

Grassley followed that tweet with another Friday morning urging Trump to follow his own advice and not get “too attached” to one policy proposal.

The letter from the five senators comes after message of RFS support sent earlier this week by the governors of Indiana, Iowa, Kansas, Missouri, Nebraska, and South Dakota. In that correspondence, the governors say they are counting on Trump “to reject any ‘RIN cap’ or waiver credit proposal to reduce RFS levels.”

Trump is also being approached from the other side of the argument and being pushed to lower the cost of RINs. Earlier this week, the International Brotherhood of Boilermakers sent Trump a letter asking him to “develop a proposal as soon as possible that brings RIN prices in line with what was intended at the outset of the program.”

The White House was set to host another industry roundtable on biofuels policy on March 12, but the administration called it off, citing “productive meetings” with Department of Agriculture and Environmental Protection Agency officials. It is unclear if and when the White House plans to convene another meeting to discuss potential changes to the RFS.