(Bloomberg) — Democrats and conservation groups are laying the groundwork to battle President-elect Donald Trump’s selection of a staunch oil industry ally and chief foe of the Obama administration’s climate agenda to lead the Environmental Protection Agency.

The nomination of Oklahoma Attorney General Scott Pruitt is "a full-fledged environmental emergency," said Sen. Brian Schatz, a Democrat from Hawaii, who pledged to make the issue a litmus test for every senator who has said they do not deny the science of climate change. "This is four-alarm fire, and we are going to do everything we can to stop his nomination."

The nomination of Pruitt, who has sued the EPA to overturn some of President Barack Obama’s signature initiatives, is a major victory for oil, gas and coal companies that have complained about a heavy hand at the agency. Pruitt relishes his role as its chief antagonist; an official biography on the 48-year-old Republican’s website proclaims him "a leading advocate against the EPA’s activist agenda."

Trump announced the Pruitt nomination on Wednesday as he nears decisions on other cabinet posts that play a critical role on energy and environmental policy. Former Texas Governor Rick Perry and Sen. Heidi Heitkamp, a Democrat from North Dakota facing a tough reelection campaign, are said to be considered as potential energy secretaries; Trump has met with Oklahoma Governor Mary Fallin and Rep. Cynthia Lummis, a Republican from Wyoming for the Interior Department post.

Rolling Back Regulations

If confirmed, Pruitt will mark the beginning of a rollback of the Obama administration’s "march to centralize energy regulation in Washington," Sen. Mike Lee, a Republican from Utah, said Thursday. "He’s spent years being ignored and pushed around by Washington, so he knows the dangerous bureaucratic mindset he’s up against," Lee said during a presentation at Heritage Foundation and Texas Public Policy Foundation energy summit. “I’m confident he will not shy away from the battle" and will be "reminding EPA regulators their job is to work with — not condescend to — the states."

The EPA has played a pivotal rule in carrying out key parts of Obama’s agenda, especially fighting climate change. Pruitt has pushed back, battling a suite of the agency’s rules in court, including a regulation slashing greenhouse gas emissions from power plants, a water pollution measure and the agency’s conclusion that carbon dioxide is a pollutant endangering public health and welfare.

He also filed the first lawsuit challenging the implementation of Obama’s health-care law.

Democratic Opposition

Several senators immediately said they would oppose his nomination, including Ed Markey, a Democrat from Massachusetts, who called Pruitt "unsuitable to lead the EPA" because of his history "carrying water for Big Oil."

"The EPA is our cop on the beat, protecting the American people and our environment from harmful pollution, hazardous waste and the impacts of climate change," Markey said in an e-mailed statement. "He has dedicated years of his career to rolling back the bedrock laws and rules that protect our water and our air."

Although cabinet nominations can no longer be filibustered in the Senate — where a simple majority is all that is needed for confirmation — Republicans will hold only a slim majority in the chamber next year, providing a possible opening to Pruitt’s Democratic foes. The nomination would be considered by the Senate Environment and Public Works Committee, set to be headed by Republican John Barrasso of Wyoming.

Sen. Sheldon Whitehouse, a Democrat from Rhode Island, laid out plans to fight the Pruitt nomination by encouraging companies with climate concerns to get involved and pressuring Republicans to vote "no." And he signaled Pruitt’s candidacy will be cast not just as devastating for the environment but also a matter of government corruption.

Environmental Setback

"When you take an agency whose primary responsibility is to deal with the climate crisis and you turn it over to someone who has spent his entire life in service to the regulated industry, that satisfies every definition of corruption in government," Whitehouse said. "That should be a theme of our efforts against him.”

No Senate Republicans have yet said they would oppose Pruitt, though Sen. Susan Collins of Maine, a prime target for the nomination’s critics, stopped short of committing to the nominee when asked Thursday.

"He’s obviously going to warrant my careful scrutiny," Collins said.

Republican Sen. Lindsey Graham of South Carolina said he’s "very inclined" to support Pruitt. "If I excluded everybody in the Republican Party who didn’t believe in climate change I wouldn’t have too many friends," Graham said.

Any Republican defections could be balanced by support from moderate Democrats in red states, lest they face recrimination at the ballot box, said Senator James Inhofe, a Republican from Oklahoma. "I think we’ll get some help from the other side," he said.

It will be an uphill fight for Democrats campaigning against Pruitt, Lee said. "The fact that they went nuclear on the executive calendar is going to make it a lot harder for them to do anything about it," Lee said, referring to the recent rules change blocking filibusters for some presidential nominations. "This is a smart guy; he’s not someone who is going to be easy for them to attack, except on the basis that they disagree with his convictions."

Confirmation Hearings

Environmental groups are urging their supporters to lobby the Senate to reject Pruitt.

“That’s a tough fight. We know that,” said Seth Gladstone, a spokesman for the Washington-based Food & Water Watch. “We will appeal to senators’ sense of moral obligation to protect the earth and human health.”

Schatz signaled Pruitt faces a tough battle.

"We are totally mobilizing on this one," he told reporters in Washington, adding that he expects "it will be extremely difficult" for Pruitt to be confirmed.

Messages left with Pruitt’s office weren’t returned.

If confirmed, Pruitt will be able to rework or rescind scores of environmental regulations officially — or simply ease off on their enforcement.

"This is about the worst thing he could be doing for the environment and the American people," said billionaire hedge-fund manager and climate activist Tom Steyer in an interview at Bloomberg headquarters in New York. Trump, "continues to appoint people who are seriously unfit and unprepared for the jobs they need to do."

The fate of Obama’s carbon rules was already uncertain under the Trump administration. The U.S. Supreme Court already halted the Clean Power Plan that aims to slash greenhouse gas emissions from power plants, pending a decision by the D.C. Circuit on the rule’s legality. Trump frequently criticized the rule on the campaign trail, promising to rip it and other "job-killing" regulations up once in office.

Clean Power

Pruitt joined 26 other attorneys general in suing to block the Clean Power Plan. Pruitt has characterized that measure, the hallmark of Obama’s climate agenda, as an affront to states’ rights and violation of the Constitution.

"The Clean Power Plan is an unlawful attempt to expand federal bureaucrats’ authority over states’ energy economies in order to shutter coal-fired power plants and eventually other sources of fossil-fuel generated electricity," Pruitt said in a statement last year.

In 2015, Pruitt also secured an injunction blocking EPA’s "Waters of the U.S." rule, which expands the scope of the Clean Water Act. Oklahoma is one of 18 states challenging that regulation in court. He also signed on to a case trying to overturn the EPA’s endangerment finding, which provided the legal underpinning for later regulation of carbon dioxide. And he has been critical of U.S. biofuel mandates, describing current policy as "unworkable."

It’s not unprecedented for an EPA administrator to have tangled with the same agency prior to leading it. For instance, Gina McCarthy, the current administrator, previously sued the EPA as a Massachusetts state official.

Congressional Allies

Pruitt’s allies in Congress, including Inhofe, lauded Trump’s decision.

"Scott has been there on every issue that we’ve had. He’s filed lawsuits that are necessary; he knows all the issues," said Inhofe, who described Pruitt is a close friend. Inhofe, who will step down as chairman of the Senate Environment and Public Works Committee next Congress but remain a senior member of the panel, predicted that Pruitt will "perform well" during his confirmation process.

That view was shared by Republican Sen. James Lankford of Oklahoma, who called Pruitt "smart on the law."

"He’s obviously argued all the way to the Supreme Court on some of the overreach of the EPA," Lankford said. "He’s very knowledgeable of what can and can’t be done by the EPA under the law."

Industry Reaction

Automakers, oil-industry leaders and electric utilities praised Pruitt’s selection as marking a change in EPA’s regulatory approach.

Mitch Bainwol, president of the Auto Alliance, which has tangled with Obama’s EPA over fuel-economy rules, called Pruitt "a terrific public servant, a great listener and a principled leader."

Chad Warmington, president of the Oklahoma Oil and Gas Association, said ,"We are not just excited about having an Oklahoman in a prominent role of a Trump administration, but we also look forward to a more measured regulatory approach at the EPA that will give a voice to all."

Pruitt has served as Oklahoma’s attorney general since 2010. In that office, he created a state "federalism unit" tasked with fighting what his website calls "unwarranted regulation and overreach by the federal government."

Keystone Filing

In May, he joined five other states in filing an amicus brief supporting TransCanada Corp.’s federal lawsuit alleging that Obama exceeded his authority in turning down a permit for the Keystone XL pipeline.

As attorney general, Pruitt has faced scrutiny over his close ties to the energy industry. In 2014, the New York Times reported that energy lobbyists had drafted letters for Pruitt to send to the EPA and other agencies challenging environmental rules that could affect the industry.

More recently, Pruitt joined Alabama Attorney General Luther Strange in criticizing other state attorney generals who are investigating whether Exxon Mobil Corp. misled investors about the business risks associated with climate change.

"Reasonable minds can disagree about the science behind global warming, and disagree they do," Pruitt and Strange said in a statement released in March. "This scientific and political debate is healthy, and it should be encouraged."

Before he was elected attorney general, Pruitt spent eight years in the state senate. In 2001, he made an unsuccessful bid to succeed Republican Steve Largent in the U.S. House of Representatives. In 2006, he sought the Republican nomination for Lieutenant Governor of Oklahoma but lost.

Bloomberg News by Jennifer A. Dlouhy and Catherine Traywick