Pruitt begins what is likely to be a controversial tenure with a clear set of goals. He has been outspoken in his view, widely shared by Republicans, that the EPA zealously overstepped its legal authority under President Barack Obama, saddling the fossil-fuel industry with unnecessary and onerous regulations.

But rolling back the environmental actions of the previous administration won’t happen quickly or easily. Even if President Trump issues executive orders aimed at undoing Obama initiatives to combat climate change, oversee waterways and wetlands and slash pollution from power plants — as he is expected to do as early as next week — existing regulations won’t disappear overnight.

To reverse or revamp existing rules around vehicle fuel standards, mercury pollution or a range of other environmental issues, Pruitt would have to repeat the lengthy bureaucratic process that generated them. Other initiatives, such as the so-called Clean Power Plan aimed at regulating emissions from power plants, remain tied up federal courts.

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In addition, Pruitt will encounter an EPA workforce on edge, in which some employees are wary about the direction he plans to take the agency and fearful he might adhere more to ideology than science. Environmental groups also are likely to oppose him at every turn, eager to sue over any rollback of existing regulations.

For his part, Pruitt has said he intends to return the agency to its central mission of protecting the quality of the nation’s air and water while respecting the role of states as primary enforcers of environmental laws.

“It is our state regulators who oftentimes best understand the local needs and the uniqueness of our environmental challenges,” he said during his confirmation hearing last month.

Pruitt cleared the Senate Friday afternoon by a vote of 52-46, winning support from Democrats Joe Manchin of West Virginia and Heidi Heitkamp of North Dakota. Only one Republican, Susan Collins of Maine, voted against him.

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The vote came after Democrats held the Senate floor for hours overnight Thursday and then through the morning to criticize Pruitt and push for a last-minute delay of his confirmation. Part of their argument centered on an Oklahoma judge’s ruling late Thursday that Pruitt’s office must turn over thousands of emails related to his communication with oil, gas and coal companies. The judge set a Tuesday deadline for release of the emails, which a nonprofit group has been seeking for years.

Republicans pressed forward with the vote, saying Pruitt had been thoroughly vetted and calling on Democrats to end what Senate Majority Leader Mitch McConnell (R-Ky.) labeled “a historic level of obstruction” in holding up Trump administration nominees.

It took only minutes after Pruitt’s confirmation to catch a glimpse of the contentious fights that lie ahead.

Environmental advocacy groups, which had written letters, lobbied lawmakers, organized protests and waged a furious campaign online and in television ads calling him a friend to polluters, reacted with a mixture of anger and despair.

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One group termed the confirmation a “sad day for the country.” Another described it the “stuff Big Oil’s dreams are made of.”

“Scott Pruitt as administrator of the EPA likely means a full-scale assault on the protections that Americans have enjoyed for clean air, clean water and a healthy climate,” Michael Brune, executive director of the Sierra Club, said in an interview. “For environmental groups, it means we’re in for the fight of our lives for the next four years.”

But amid such hand-wringing, there was relief among those who welcomed his nomination — a group that includes fossil-fuel firms that chafed under the regulation of the Obama era. Many have helped fund Pruitt’s campaigns over the years.

The National Association of Manufacturers proclaimed Pruitt would “restore balance to the way environmental regulations are developed.” The head of the National Mining Association said he will be “mindful of the costs that regulations can impose on the economy.”

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The White House itself rejoiced at Pruitt’s confirmation, with spokeswoman Sarah Huckabee Sanders telling reporters aboard Air Force One that “the EPA will no longer spend unnecessary taxpayer dollars on an out-of-control, anti-energy agenda.”

Jeff Holmstead, who headed EPA’s air and radiation office under President George W. Bush and is now a lawyer representing energy firms, said he thinks Pruitt will be a good steward of the agency.

“Over the past eight years in particular, [the EPA] has completely micromanaged the states. I think you’ll see a real effort to reset that balance,” Holmstead said. “I think he really does believe in the rule of the law. He believes the role of executive branch is to carry out the intent of Congress.”

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Pruitt sued the EPA more than a dozen times during the Obama administration, challenging the agency’s authority to regulate toxic mercury pollution, smog, carbon emissions from power plants and the quality of wetlands and other waters. In Oklahoma, he dismantled a specialized environmental protection unit that had existed under his Democratic predecessor and established a “federalism unit” to combat what he called “unwarranted regulation and systematic overreach” by Washington.

That combative approach won him praise from fellow Republicans and the oil and gas industry. But the prospect of Pruitt leading the EPA horrified environmental advocates, who accuse him of repeatedly questioning the overwhelming scientific consensus around climate change and defending the interests of fossil-fuel firms over the health of ordinary citizens.

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His nomination also rattled some agency employees, who fear he will be eager to carry out the promise that Trump made on the campaign trail to “get rid of [EPA] in almost every form.We’re going to have little tidbits left, but we’re going to take a tremendous amount out.”

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“Unless he has a revelation like St. Paul did on road to Damascus, I don’t anticipate anything good,” said John O’Grady, who heads a national council of EPA unions and has worked at the agency for more than 30 years.

More than 700 former EPA officials recently wrote to Congress opposing Pruitt’s confirmation, saying he “has gone to disturbing lengths to advance the views and interests of business.” Even some current employees openly protested his nomination, notably during a recent rally in downtown Chicago near the agency’s Region 5 offices.

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Minutes after Friday’s confirmation, the EPA tweeted for the first time since Trump’s inauguration. “We’d like to congratulate Mr. Pruitt on his confirmation!” the tweet read.

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Soon after, the agency issued its first press releases since Trump became president. One included praise for Pruitt from more than a dozen Republican lawmakers and industry executives. The EPA also posted an online biography for its new administrator, which made no mention of the many lawsuits Pruitt had filed against the agency he now leads.

Susan Hogan and Sean Sullivan contributed to this report.

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