WASHINGTON (AP) — A Senate committee has delayed consideration of four of President Donald Trump’s nominees for key posts at the Environmental Protection Agency amid opposition to the administration’s proposed reduction in the volume of biofuels blended into gasoline and diesel.

A meeting of the Senate Committee on Environment and Public Works set for Wednesday was postponed after a bipartisan group of 33 senators urged EPA Administrator Scott Pruitt to change his position on the Renewable Fuel Standard. Pruitt has proposed targets for 2017 and 2018 set slightly below current levels following a push by oil companies to ease mandates on using ethanol from corn and soybeans.

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Three of the 11 Republicans on the committee signed the letter urging Pruitt to change his position. With Democrats saying they will oppose Trump’s picks, the defection of just one of the committee’s Republicans could torpedo a nominee.

GOP Sen. Joni Ernst of Iowa said Wednesday she was not prepared to support the nomination of Bill Wehrum, Trump’s choice to lead the EPA’s Office of Air and Radiation. If confirmed, Wehrum would oversee the Renewable Fuel Standard.

Ernst questioned Wehrum about his position on the biofuels mandate in a hearing earlier this month.

“Following his confirmation hearing, I expressed concern with the answers he gave, and told the committee I wasn’t comfortable supporting him at that time due to his answers and recent actions by the EPA which had the potential to weaken or undermine the RFS,” Ernst said.

Wehrum is a lawyer in private practice whose clients have included pro-fossil fuel groups opposed to mandated reductions of planet-warming carbon emissions — including the American Petroleum Institute, the American Fuel and Petrochemical Manufacturers and the American Chemistry Council.

Democratic Sen. Tammy Duckworth of Illinois said Wednesday she would place a hold on the nominations of Wehrum and Michael Dourson, Trump’s pick to lead EPA’s Office of Chemical Safety and Pollution Prevention.

A hold is an informal practice by which a senator can delay, at least temporarily, a nomination or piece of legislation from coming to a vote by signaling that he or she may filibuster any motion to consider the measure.

The Associated Press and other media outlets reported last month that Dourson has for years accepted payments for criticizing studies that raised concerns about the safety of his clients’ products. Past corporate clients of Dourson and a research group he ran include Dow Chemical Co., Koch Industries Inc. and Chevron Corp.

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A staunch supporter of the biofuels program, Duckworth said she had seen firsthand the price the nation pays for its dependence on oil imported from foreign adversaries. The retired U.S. Army lieutenant colonel lost her legs during the Iraq War when her Black Hawk helicopter was brought down by enemy fire.

“Mr. Wehrum’s history of attacking the biofuels industry and his refusal to recuse himself from RFS-related issues despite his well-documented conflicts of interest should alarm all of my colleagues,” Duckworth said. “Any senator who supports the RFS program, our farmers and our commitment to the environment and energy dependence must oppose his nomination.”

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Follow Associated Press environmental reporter Michael Biesecker at http://twitter.com/mbieseck