President Donald Trump’s pick to run the Environmental Protection Agency — former coal-industry lobbyist Andrew Wheeler — has faced considerable criticism from Democrats.

He’s feeling pressure from the other side of the aisle as well, with Texas Sen. Ted Cruz and four other Republican senators threatening to oppose his nomination.

In a letter this week, Cruz and his colleagues expressed concerns about the costs that oil refiners face due to a requirement known as the Renewable Fuel Standard, which aims to boost the use of corn-based ethanol or other biofuels.

“As we continue to evaluate your nomination to be Administrator, it is important that we have a better understanding of your views and approach to administering the RFS and the agency actions you believe could alleviate costs,” said the letter, which was posted online by Bloomberg News.

“Without an adequate proposal to meaningfully lower the regulatory burden ... we will have serious concerns with your nomination,” added Cruz, Pennsylvania Sen. Pat Toomey, Utah Sen. Michael Lee and Louisiana’s two senators, John Kennedy and Bill Cassidy.

Ethanol’s supporters also are writing letters to Wheeler, who already has been leading the EPA as acting administrator. Geoff Cooper, president and CEO of the Renewable Fuels Association, which lobbies for the ethanol industry, highlighted his trade group’s missive in a speech this week at its annual conference.

“When the EPA releases its proposal to reset the 2020 through 2022 RFS volumes, we’ll be primed to make the case that the reset should be used to increase the required volumes of all renewable fuels over current levels,” Cooper said, according to his prepared remarks.

“In fact, as we wrote to EPA Administrator Andrew Wheeler just two weeks ago, the reset rule presents a perfect opportunity for the Agency to restore the conventional renewable volumes that were inappropriately erased by Scott Pruitt’s small-refinery waivers and the Obama administration’s illegal use of a ‘general waiver,’” he added. Pruitt was Wheeler’s controversial predecessor.

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Wheeler is also hearing from the ethanol industry’s supporters on another key issue — its upcoming new rule for E15, a gasoline blend that’s 15% ethanol. “I urge you to publically commit to completing this rule before June,” said Democratic Sen. Dick Durbin of Illinois, a Corn Belt state, in a letter this week to the EPA official. Durbin’s letter noted that President Donald Trump has committed to ending summertime restrictions on E15, but the recent partial government shutdown slowed the EPA’s rulemaking process on the issue.

Durbin’s campaign committee and leadership PAC have received $134,000 from agribusiness sources in the past five years, according to data from OpenSecrets.org, a website tracking money in politics that’s run by a nonpartisan research group, the Center for Responsive Politics

Meanwhile, Cruz’s campaign committee and leadership PAC have received $1.7 million from oil and gas industry sources in the past five years, according to OpenSecrets.org’s data. All five GOP senators that wrote the letter about RFS are from states that host oil refineries, a Reuters report noted.