Among Donald Trump's cabinet nominees awaiting Senate confirmation, few are expected to face tougher scrutiny than Scott Pruitt, the president-elect's pick to head the Environmental Protection Agency. The Oklahoma attorney general is a relentless adversary of that agency and its regulations, and he has deep ties to the fossil fuel industry he would be charged with overseeing.

In five years as chief law enforcer for his energy-rich state, Pruitt has transformed his office into a regiment in the war against federal regulation of industry. He has been a party to at least 14 lawsuits challenging the EPA's authority to impose rules on oil and gas production, vehicles and coal-fired electricity, helping to lead the campaign against President Barack Obama's signature climate initiative, the Clean Power Plan.

Pruitt will face tough questions, mostly from Democrats on the Senate Environment and Public Works Committee, the panel weighing his nomination on Wednesday, on whether the fossil fuel industry's generous support for his political career would compromise his leadership of the EPA.

Among the evidence to that end:

His 2011 letter urging the EPA to drop its efforts to reduce methane emissions, later shown to have been written by Oklahoma-based fracking firm Devon Energy, a campaign donor.

His leadership of a nonprofit group, the Rule of Law Defense Fund, funded in part by the Koch brothers, petrochemical billionaires who lobby heavily against environmental regulations.

His ties to a super-PAC, Liberty 2.0, which its officials agreed to close last week after questions were raised about the unprecedented opportunity it created for industry interests to make unlimited financial contributions to support a sitting EPA administrator.

His involvement in an unsuccessful lawsuit that sought to overturn the EPA's finding that greenhouse gases pose a danger to human health and the environment, the underpinning of every action that the agency has taken to address carbon dioxide emissions.

While opponents of Pruitt face an uphill battle, Democrats on the Senate environmental panel have made clear their plan to hit the nominee hard with questions on conflicts of interest. They sent letters last week to the Office of Government Ethics and the EPA's chief ethics officer requesting opinions on Pruitt's obligations to disclose hidden funders to the nonprofit he led. They also asked about ethics issues raised by his litigation in cases joined by industry groups from whom he has solicited money.

The stakes over Pruitt's nomination are high for environmental advocates and their Democratic allies. They fear the Trump administration will embark on an historic rollback of 40 years of federal protections for air, water and land, as well as dismantling the first federal limits on global warming emissions.

Neither Pruitt's counsel nor the Trump transition team responded to queries on the conflict of interest questions. But Pruitt has defended his work on behalf of Devon Energy, first detailed in a Pulitzer Prize-winning series by The New York Times in 2014.

"A constituent, an Oklahoma company," he told NPR's StateImpact Oklahoma, referring to Devon, "made us aware of the overreach of the BLM (Bureau of Land Management) around something as important as hydraulic fracturing" in 2014. "That's actually called representative government in my view of the world....I don't think there's anything secretive in what we've done."

But some of Pruitt's relationships with industry groups have been shrouded, including the funding behind the Rule of Law Defense Fund (RLDF), where he was a board chair from November 2015 to November 2016. As a social welfare nonprofit, it doesn't have to disclose its donors.

But tax filings of the Koch brothers' Freedom Partners Foundation show that it contributed $175,000 of the group's $885,000 in revenue in 2014. And RLDF is an affiliate of the Republican Attorneys General Association, which has received more than $2.25 million in funding from fossil fuel interests since 2015—money that goes primarily to help elect GOP attorneys general, according to an analysis of its disclosures by the Center for Media and Democracy.

That group obtained a confidential agenda of an RLDF retreat in April 2016, where Pruitt spoke on the fight against the Clean Power Plan. With that fight headed to the Supreme Court, Pruitt apparently was very concerned with the court's makeup. CMD also unearthed letters showing that Pruitt coordinated fellow state AGs to oppose the nomination of U.S. District Judge Merrick Garland to fill the vacancy left by the death of Justice Antonin Scalia. He touted the effort in a fundraising letter he sent to a political action committee called Oklahoma Strong, set up by his campaign workers to raise money for other candidates.

Six Democratic members of the Senate environment committee have signed a letter to Pruitt asking for the names of donors to RLDF, as well as details of the group's work. He has not responded.

Senator Sheldon Whitehouse (D-R.I.), one of the letter's signers, said it was important to get full disclosure, because of the history of what he called the "sophisticated dark money operation" of the fossil fuel industry. "That operation's sole purpose is to advance the interests of the fossil fuel industry without any regard for the health and wellbeing of the American people," he said in an email. "That's why it's so important to know how beholden the EPA nominee and his dark money groups are to the oil and gas giants."

The Democratic senators "are leveling absurd politically motivated attacks against a non profit policy organization that advocates for defense of our Constitution and the rule of law in a wide range of policy issues," Jordan Russell, spokesman for RLDF, wrote in an email.

Pruitt's supporters say his environmental record includes cases to clean up water pollution and against the oil industry for overbilling the state's clean-up fund for leaking underground storage tanks. They say Pruitt sued the EPA because he believes the agency has exceeded its legal authority in imposing mandates on the states. "Based on his record, I expect that we would see an EPA that returns to an emphasis on cooperative federalism," said William Yeatman, senior fellow for environmental issues at the conservative Competitive Enterprise Institute.

The fossil fuel industry has helped underwrite Pruitt's political career, with Continental Resources CEO Harold Hamm serving as honorary chairman of his 2014 election campaign, and oil, gas and coal interests contributing nearly half of the $450,000 raised in the past two years for Liberty 2.0. Both Liberty 2.0 and Oklahoma Strong were established in 2015 in his campaign office, although Liberty 2.0 is considered independent from the candidate, which allowed it to solicit unlimited donations. Earlier this week, an attorney for the group announced the Super PAC would shut down.

"We have heard Donald Trump say over and over again, 'Drain the swamp!,' but Pruitt is the opposite of that," said Tiernan Sittenfeld, senior vice president for government affairs for the League of Conservation Voters (LCV). "The pay-to-play is just beyond the pale."

Rhea Suh, president of the Natural Resources Defense Council, called Pruitt "one of the most notorious nominees in the agency's history, a man who has made an entire career out of blocking EPA's ability to do its job."

Green groups are urging their members to call their senators to oppose Pruitt.

Clean Air Moms Action, a project of EDF Action, is focusing on swaying four Republican senators to vote against him, running anti-Pruitt TV ads that seem designed to appeal to conservatives by borrowing fetal heartbeat imagery from anti-abortion ads. The ads show a sonogram, while highlighting a legal brief that Pruitt signed challenging EPA rules to control coal plant emissions of the neurotoxin mercury. "Scott Pruitt actually questions whether mercury poses a public health hazard," the ad says. Meanwhile, the Sierra Club is lampooning the nominee as #PollutingPruitt on social media.

Pruitt's supporters are fighting back hard. The drive is being spearheaded by America Rising Advanced Research (also known as AR Squared), a Republican opposition research group, and FreedomWorks, the conservative group that helped launch the Tea Party movement. They are targeting ads and a phone call blitz at five Democratic senators who are up for reelection in 2018 in states Trump won. AR Squared's web site, ConfirmPruitt.com, already trumpeted a win in its campaign: coal state Sen. Joe Manchin (D-W.Va.), signaled he will break ranks with his party and support Pruitt.

With Manchin's defection, at least four Republican votes would be needed to defeat Pruitt in a Senate where Republicans enjoy a 52-48 advantage. Among the GOP votes environmentalists are trying to sway are those who have voiced previous support for climate change legislation, including Susan Collins of Maine, Lindsey Graham of South Carolina and Lamar Alexander of Tennessee. Another Republican who previously sponsored climate legislation, John McCain of Arizona, has already expressed support for Pruitt. But hedge fund billionaire activist Tom Steyer's group, NextGen Climate, hasn't given up on his vote—rolling out a TV ad campaign last weekend that included Arizona.

Environmental groups are also trying to win the vote of Sen. Dean Heller of Nevada, a state that Trump lost and where support for renewable energy is strong. But there's concern among Pruitt opponents about losing more Democratic votes. EDF Action is targeting its TV ads not only at Republicans but also at three Democrats from states that Trump won: Bob Casey of Pennsylvania, Sherrod Brown of Ohio and Bill Nelson of Florida.

"Regardless of whatever the odds may be, we will use each and every one of these opportunities to make the case to the American public of what's at stake," said Suh of NRDC in a press call earlier this month. "It gives us a chance to talk about what's at stake to the American public, to talk about the importance of these agencies, and the relevance of these agencies to everyday people."