Jabin Botsford/The Washington Post via Getty Images Oklahoma Attorney General Scott Pruitt, pictured arriving at Trump Tower in New York on Dec. 7, 2016, has been a major critic of the Environmental Protection Agency.

Six senators urged President-elect Donald Trump’s Environmental Protection Agency pick on Wednesday to reveal his ties to a secretive fundraising group that enables fossil fuel companies to make anonymous donations to politicians.

Oklahoma Attorney General Scott Pruitt helped found the nonprofit Rule of Law Defense Fund to collect campaign contributions from companies like oil giant ConocoPhillips and coal mining titan Alpha Natural Resources, The New York Times reported in December 2014. Pruitt now sits on the group’s board.

The group, which keeps the identities of most of its donors secret, received $175,000 in 2014 from Freedom Partners, a business organization that organizes political activities for the billionaire brothers Charles and David Koch, who run the privately held chemical and oil giant Koch Industries.

“Before the Senate votes to confirm you to run EPA, it is important that you provide a full disclosure of your relationship with the energy industry so we can determine how that will influence your ability to run the agency,” read a letter signed by Sen. Bernie Sanders (I-Vt.), Sen. Sheldon Whitehouse (D-R.I.), Sen. Edward Markey (D-Mass.), Sen. Jeffrey Merkley (D-Ore.), Sen. Cory Booker (D-N.J.) and Sen. Benjamin Cardin (D-Md.). The senators are all members of the Senate Committee on Environment and Public Works.

Sen. Jim Inhofe (R-Okla.), the committee’s chair who last year infamously displayed a snowball on the Senate floor as evidence of climate change being a hoax, derided the letter as a “baseless political stunt.” The Rule of Law Defense Fund denounced the letter as “partisan obstructionism” and called on Pruitt to dismiss it as having “no legal significance.”

“Aside from disrespectfully lecturing you about the purpose of the agency you have been nominated to lead, and with which you are acutely familiar, the Democrats seek information about RLDF that they are not entitled to under the law, including confidential donor information,” Charles R. Spies and James E. Tyrrell III, lawyers representing the Rule of Law Defense Fund, wrote in a letter to Pruitt later on Wednesday. “The Democrats’ letter is transparently a partisan fishing expedition intended to smear your confirmation process and gin up fundraising support from radical, far-left environmentalist groups.”

Pruitt’s ties to fossil fuel companies run deep, making him “too extreme,” the Denver Post declared in an editorial, to lead an agency tasked with defending the environment from polluters. He led the charge to block President Barack Obama from implementing his Clean Power Plan, a set of new EPA regulations aimed at cutting carbon emissions from utility companies.

The 48-year-old once signed his name to a letter to the EPA, authored by lawyers of a major oil and gas company in his state, that criticized estimates on emissions from natural gas drilling sites.

After publishing 84 pages of correspondence in 2014 between Pruitt and the company, Devon Energy, The New York Times called the communications evidence of an “unprecedented, secretive alliance that Mr. Pruitt and other Republican attorneys general have formed with some of the nation’s top energy producers to push back against the Obama regulatory agenda.”

“We have been troubled that as Attorney General of Oklahoma you used, nearly verbatim, industry talking points in official correspondence your office sent to EPA concerning EPA’s estimation of methane pollution in your state,” the senators wrote in the letter. “Your relationship with the Rule of Law Defense Fund (the Fund) has received less attention but is no less troubling.”

This year, Pruitt defended Exxon Mobil Corp. as it fought off Democratic state attorneys generals’ probes into its history of funding a disinformation campaign that questioned fossil fuels’ role in global warming. In May, he wrote an op-ed criticizing the attorneys general as a “climate-change gang” just three weeks after Exxon Mobil donated $50,000 to an association Pruitt led for two years, The Huffington Post reported last week.

Pruitt is even suing to block an EPA rule to protect streams and wetlands under the 1972 Clean Water Act.

“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 senators wrote. “Accordingly, we look forward to your timely response to this request.”

This story has been updated with comments from Sen. Jim Inhofe and the Rule of Law Defense Fund.

Read the senators’ full letter below.