A group of bipartisan lawmakers on Tuesday asked the Environmental Protection Agency to stop issuing waivers that allow oil refiners to avoid obligations to blend biofuels into gasoline.

The 13 senators, led by Iowa Republican Chuck Grassley and Minnesota Democrat Amy Klobuchar, also requested in a letter that EPA provide justification for why the agency has allowed an unusual number of large, profitable refiners to sidestep the biofuels mandate.

The letter is the latest salvo in a battle between two pillars of President Donald Trump's political base over the so-called Renewable Fuel Standard. On one side are farm states, who want to protect corn-based ethanol's role in the nation's energy mix. On the other side are energy companies who says the biofuels rules risk the nation's energy security by threatening to tip refiners into bankruptcy.

The Trump EPA has reportedly issued 25 of the "hardship" waivers, usually reserved for small, distressed refiners who cannot meet their obligation to either blend biofuels into petroleum products or purchase credits from competitors who can.

The lawmakers also requested a list of refiners who have received waivers since 2016. The EPA has declined to make the names public, saying it would amount to disclosing private company information.

The waiver recipients include the smallest refineries operated by energy giants ExxonMobil and Chevron, Reuters reported earlier this month. Three out of ten refineries operated by one of the nation's biggest refiners, Andeavor, have also received exemptions, according to Reuters.

"Recent reports indicate dozens of small refiner waivers have been secretly granted to large, multibillion-dollar companies under the guise of the small refinery hardship exemption provision in section 211(o)(9) of the Clean Air Act. This is extremely concerning to us," the senators wrote in a letter to EPA Administrator Scott Pruitt.

The senators also want to know whether EPA is forcing other refiners to blend enough biofuel to offset the volumes it has waived in recent months. They also asked EPA to commit to issuing future waivers only during an annual rulemaking process, when the public can comment on proposed exemptions.

In response to the letter, EPA spokesperson Liz Bowman said, "Discontinuing the issuance of small refinery hardship waivers would be a violation of the Renewable Fuel Standard; as EPA doesn't have the authority to pick and choose which provisions of the RFS to follow."

"The Agency has been both responsive and open about small refinery waivers, while being careful not to violate confidential business information protections," Bowman said in an email.

Waivers "undermine" Trump's commitment

The senators say the recent waivers "undermine commitments President Trump made on the Renewable Fuel Standard (RFS) to our constituents."

Trump said he backed the program on the campaign trail, but disappointed both sides of the debate in December when he left the current biofuels requirements largely unchanged. The ethanol industry and heartland lawmakers hoped EPA would strengthen advanced biofuels mandates, while oil interests and conservative think tanks wanted the program to be scrapped.

Opponents and supporters of the program have recently attempted to negotiate reforms to the program. However, Grassley and his fellow Iowa Republican, Sen. Joni Ernst, rejected proposals supported by Pruitt, Agriculture Senator Sonny Perdue and GOP Sen. Ted Cruz of Texas.

The other senators who signed the letter were Joni Ernst (R-Iowa), Debbie Stabenow (R-Mich.), Deb Fischer (R-Neb.), Dick Durbin (D-Ill.), John Thune (R-S.D.), Tina Smith (D-Minn.), Roy Blunt (R-Mo.), Claire McCaskill (D-Mo.), Tammy Duckworth (D-Ill.), Heidi Heitkamp (D-N.D.) and Joe Donnelly (D-Ind.