(CNN) Iowa Republican Sen. Chuck Grassley warned Tuesday he may call on Environmental Protection Agency Administrator Scott Pruitt to resign if his agency does not take a fairer approach to issuing waivers to the large refineries that allow them to skirt biofuels regulations.

Grassley, who chairs the influential Senate Judiciary Committee, made the comments on a conference call with agriculture reporters on Tuesday where he was asked about the EPA issuing waivers. He said the EPA needs to take a fairer approach "or I'm going to be calling for Pruitt to resign because I'm done playing around with this."

Grassley's criticism is significant given his position as a high-profile Republican senator, but the issue of the EPA waivers has not gotten as much attention compared to the ethics watchdogs, federal auditors and congressional committees who are already conducting nearly a dozen inquiries into Pruitt's actions at the agency.