President Donald Trump's pick to oversee chemical safety at the Environmental Protection Agency withdrew his nomination Wednesday after bipartisan opposition made his Senate confirmation unlikely.

Officials at the White House and the Senate told The Associated Press that Michael Dourson had sent a letter asking his name to be removed from consideration to serve as head of the EPA's Office of Chemical Safety and Pollution Prevention. The two officials were not authorized to discuss the matter publicly and spoke on condition of anonymity.

North Carolina's two Republican senators, Richard Burr and Thom Tillis, said last month they would vote against Dourson's nomination after The Associated Press and other media outlets detailed his past work as a toxicologist hired to defend major chemical companies.

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The Senate's 48 Democrats were united in opposition, meaning only one more GOP defection would be needed to defeat Dourson's nomination.

In his letter asking the president to withdraw his name from consideration, which was obtained by the AP, Dourson said his stepping aside "avoids unnecessarily politicizing the important environmental protection goals of Administrator Pruitt."

"I sincerely and deeply appreciate all the love and support by my family, friends and colleagues during this 'surprising' confirmation process," he went on to say.

Sen. Tom Carper of Delaware, the top Democrat on the Senate Environment and Public Works Committee, said his staunch objections to Dourson's nomination were never a matter of partisanship.

"I sincerely believe he is the wrong person to hold this important position, and it's become clear that, even with a Republican majority in the Senate, he could not be confirmed," Carper said. "Dourson, an individual who has spent most of his career promoting less protective chemical safety standards, had no business overseeing our nation's chemical safety laws."

The EPA's press office did not respond to requests for comment Wednesday evening. Dourson has already been serving at the agency as a senior adviser to EPA administrator Scott Pruitt. It was not immediately clear whether he will continue in that role, which does not require Senate confirmation.

The AP reported in September that Dourson has for years accepted payments for criticizing studies that raised concerns about the safety of his clients' products, according to a review of financial records and his published work.

Past corporate clients of Dourson and of a research group he ran include Dow Chemical Co., Koch Industries Inc. and Chevron Corp. His research has also been underwritten by industry trade and lobbying groups representing the makers of plastics, pesticides, processed foods and cigarettes.

Burr and Tillis, both of whom are considered reliably pro-business conservatives, cited Dourson's past work and worries among their home-state constituents about tainted drinking water in opposing his nomination.

Marine veterans and their families have blamed decades-old contamination of wells at a North Carolina base with solvents and dry-cleaning chemicals for infant deaths and serious health problems, including cancer.

More recently, concerns have been raised about undisclosed discharges of chemicals used to manufacture Teflon and GoreTex into the Cape Fear River, a source of municipal drinking water for Wilmington and other southeastern North Carolina communities.

Dourson worked at the EPA for more than a decade, leaving in 1994 as the manager at a lab that assessed the health risks of exposure to chemicals. The following year, he founded Toxicology Excellence for Risk Assessment, a private, nonprofit organization that tests chemicals and produces reports on which chemicals are hazardous in what quantities.

Dourson's views toward industry are consistent with others Trump has selected as top federal regulators. Among them is Pruitt, who in March overruled the findings of his agency's own scientists to reverse an effort to ban chlorpyrifos, one of the nation's most widely used pesticides.

Court records show Dourson and his work have also often been called on when his corporate clients are seeking to fend off lawsuits.

Dourson's withdrawal was first reported Wednesday by Bloomberg News.