Scott Pruitt—Donald Trump's controversial pick to head the U.S. Environmental Protection Agency (EPA)—is under fire for his supposed ties to a nonprofit controlled by billionaire oil tycoons, Charles and David Koch.

Scott Pruitt, Donald Trump's nominee to head the EPA, has been known to parrot oil and gas industry talking points.Flickr

POLITICO has received a copy of a letter sent to Pruitt that was signed by six Democratic senators on the Environment and Public Works Committee. The letter raises questions about the Oklahoma attorney general's director status at a nonprofit called the Rule of Law Defense Fund that accepted $175,000 from Freedom Partners, the political arm of the Koch brothers' network.

Freedom Partners has been described as a "dark money umbrella group," likened to a secret bank that disburses contributions from wealthy conservatives.

The senators are requesting names of donors, meeting information, internal emails and other details related to Pruitt's leading role at the Defense Fund, according to POLITICO.

The letter was signed by Sens. Sheldon Whitehouse of Rhode Island, Jeff Merkley of Oregon, Cory Booker of New Jersey, Ed Markey of Massachusetts, Ben Cardin of Maryland and Bernie Sanders (I-Vt.), who caucuses with Democrats.

As POLITICO noted, the Defense Fund is allowed to keep donors secret since it is organized under a special section of the tax code. However, in 2014, it received $175,000 from Freedom Partners.

The letter calls that contribution into question and said that Pruitt's work with the Defense Fund is "troubling," since he may be too close to the very fossil fuel companies he is supposed to regulate as Trump's EPA chief.

The Defense Fund is an offshoot of the Republican Attorneys General Association, itself a group that has deep ties to the fossil fuel industry. A December 2014 New York Times exposé revealed an "unprecedented, secretive alliance" and coordination between Big Oil and the association.

Trump's appointment of Pruitt was met with unprecedented criticism by environmental and health organizations nationwide, being described as a "puppet" of the fossil fuel industry. In 2014, Pruitt was caught sending letters on state government letterheads to President Obama and federal agency heads asserting that the EPA was overestimating the air pollution from drilling for natural gas in Oklahoma. Turns out, the letter was written by lawyers for one of the state's largest oil and gas companies, Devon Energy.

According to POLITICO, the six senators are highlighting Pruitt's links to fossil fuel interests as part of a larger effort to turn moderate senators against his confirmation.

"The confirmation process, starting with your responses to committee questions before your hearing, is an opportunity for you to dispel the notion that the advocacy you have undertaken on environmental issues as Attorney General of Oklahoma has been directed by and for the benefit of the energy industry," the letter states.

As Oklahoma's top legal officer, Pruitt has waged numerous legal wars against the EPA and President Obama's environmental regulations, including the president's signature Clean Power Plan. Trump said Pruitt is "highly respected" and will counter the EPA's "anti-energy agenda that has destroyed millions of jobs."