The courts, for instance, found that the E.P.A. had ignored clear legal statutes when they ruled that Mr. Pruitt had illegally delayed a regulation curbing methane emissions from new oil and gas wells and that the agency had broken the law by missing a deadline last year to enact ozone restrictions.
In other cases — including one in which a federal court ordered the E.P.A. to act on a Connecticut request to reduce pollution from a Pennsylvania power plant, and one where judges demanded quick action from the agency on new lead paint standards — the courts warned Mr. Pruitt that avoiding enacting regulations already on the books was an inappropriate effort to repeal a rule without justifying the action.
“The E.P.A. has a clear duty to act,” a panel of judges of the San Francisco-based Court of Appeals for the 9th Circuit wrote in a 2-1 decision finding that the agency must revise its lead paint standards in 90 days, as regulations required. The agency had tried to delay the revisions for six years.
In an interview on Friday, the White House spokeswoman, Sarah Huckabee Sanders, said that Mr. Trump felt that Mr. Pruitt had done a satisfactory job at the EPA. Her comments suggested that Mr. Pruitt’s work checking off items on the president’s agenda — including rolling back a large number of environmental protections — may weigh heavily as a counterbalance to the ethics questions related to his travel expenses, management practices and his rental of a living space from the wife of a prominent lobbyist.
Describing Mr. Trump’s view of Mr. Pruitt, she said: “He likes the work product.”
Liz Bowman, an E.P.A. spokeswoman, disputed the criticisms of the agency’s work. “E.P.A. does its due diligence, consults with O.M.B. and other federal agencies to ensure that its work is legally defensible,” she said in an email, referring to the Office of Management and Budget, the office that coordinates and evaluates policy across the executive branch.
One of the chief examples cited by Mr. Pruitt’s critics came this week when the E.P.A. filed its legal justification for what is arguably the largest rollback of an environmental rule in the Trump administration: the proposed undoing of an Obama-era regulation aimed at cutting pollution of planet-warming greenhouse gases from vehicle tailpipes.
Mr. Pruitt made his case for the rollback in a 38-page document filed on Tuesday that, experts say, was devoid of the kind of supporting legal, scientific and technical data that courts have shown they expect to see when considering challenges to regulatory changes.
“There’s an incredible lack of numbers,” said James McCargar, a former senior policy analyst at the E.P.A. who worked on vehicle emissions programs and remains in close touch with career staffers who work on those programs. “If this gets challenged in court, I just don’t see how they provide anything that gives a technical justification to undo the rule.”
The rules Mr. Pruitt is targeting would require automakers to nearly double the average fuel economy of passenger vehicles to 54.5 miles per gallon by 2025. Automakers have argued the rule is onerous, forcing them to invest heavily in building hybrid and electric vehicles.
As part of the process, Mr. Pruitt filed the 38-page document, which is meant to supply the government’s legal justification for rolling back the rule. About half the document consists of quotations from automakers laying out their objections to the rule. By comparison, the Obama administration’s 1,217-page document justifying its implementation of the regulation included technical, scientific and economic analyses justifying the rule.
Experts in environmental policy said the lack of analytical arguments in this week’s E.P.A. filing surprised them. “This document is unprecedented,” said Mr. McCargar, the former E.P.A. senior policy analyst. “The E.P.A. has just never done anything like this.”
John M. DeCicco, a professor of engineering and public policy at the University of Michigan Energy Institute, said the filing was a departure from the practices of previous Republican and Democratic administrations.
“A president or an administrator or somebody can’t just say, ‘I’m going to change the rule,’ without justifying it very, very carefully,” Mr. DeCicco said. “As a scientist who’s worked on these issues, I’m saying, where are the numbers? Where’s the data?”
Most of the document consists of arguments quoting directly from public comments made by automaker lobbyists, the Alliance of Automobile Manufacturers and the Global Automakers, that the pollution rules will be unduly burdensome on the auto industry, as well as public comments from Toyota, Fiat Chrysler, Mercedes-Benz and Mitsubishi.
While it does include arguments opposing the regulatory rollback from groups including the Union of Concerned Scientists and the state of California, it does not contain what environmental experts say is the critical element of a legally strong justification for changing an E.P.A. regulation: Technical analysis of both sides of the argument leading to a conclusion aimed at persuading a judge that the change is defensible.
Seth Michaels, a spokesman for the Union of Concerned Scientists, suggested that, in its reuse of arguments by the automakers’ lobby, the emissions-rollback document echoed Mr. Pruitt’s modus operandi when he was the Oklahoma Attorney General.
“It’s reminiscent of the 2011 letter Scott Pruitt sent as Oklahoma AG to the E.P.A., in which he took a letter drafted by lawyers for Devon Energy and stuck his name on it with minimal edits,” Mr. Michaels said.
A 2014 investigation by The Times found that lobbyists for Devon Energy, an Oklahoma oil and gas company, drafted letters for Mr. Pruitt to send to the E.P.A., the Interior Department, the Office of Management and Budget and President Obama, outlining the economic hardship of various environmental rules.
Between 2011 and 2017, Mr. Pruitt filed suit against the E.P.A. 14 times, and lost almost all of the cases.
Most were filed in conjunction with the Republican attorneys general of a dozen or more other states, making it difficult to know precisely which legal arguments his office contributed, legal experts said. Mr. Pruitt frequently took a lead role in the cases.
In the end, “a lot of those arguments were losers,” said Richard L. Revesz, an expert in environmental law at New York University.
In particular, Mr. Revesz noted a case brought by the group against President Obama’s signature climate change regulation, the Clean Power Plan, which Mr. Pruitt is now working to overturn from within the E.P.A. The lawsuit challenged a draft proposal of the regulation, which was an unprecedented move that a federal court quickly struck down, saying that they could not legally challenge a draft.
While the attorneys general, including Mr. Pruitt, garnered media attention for the case, “The argument they had was ludicrous,” Mr. Revesz said.
The group did, however, score one major victory: After the Obama administration issued its final version of the Clean Power Plan, it successfully petitioned the Supreme Court to temporarily halt implementation of the rule.
Since taking the helm of E.P.A., Mr. Pruitt has barnstormed the country, meeting with farmers, coal miners and local leaders and promising an end to his predecessor’s regulatory approach. He also has favored closed-door policy speeches to conservative think tanks, like the Heritage Foundation, to roll out policy initiatives.
The Heritage Foundation was the venue Mr. Pruitt chose this year to say that he would make changes to how scientific studies are considered at the agency. Both critics and supporters of Mr. Pruitt said that, by making the proposal in a political fashion rather than changing the rules in a quieter but potentially more lasting way means that changes like these are more vulnerable to being undone by a future administration.
Environmental groups have welcomed Mr. Pruitt’s court losses. Joanne Spalding, chief climate counsel for the Sierra Club, said she was pleased by what she called “sloppy” and “careless” E.P.A. legal work. “It’s fine with us,” she said. “Do a bad job repealing these things, because then we get to go to court and win.”
Thomas J. Pyle, a supporter of Mr. Pruitt’s and the president of the Institute for Energy Research, a think tank that promotes fossil fuels, described that as spin. “The environmental left portrays Scott Pruitt as a devil incarnate in their fund-raising solicitations, yet brag about how ineffective he is in dismantling Obama’s climate rules,” he said. “Which is it?”
Still, some conservatives said they were worried that Mr. Pruitt was more interested in media attention than policy and feared more legal losses. “If the goal is to generate temporary relief and to make a splash, then what they’re doing is terrifically fine,” said Jonathan H. Adler, director of the Center for Business Law & Regulation at Case Western Reserve University School of Law.
But if the Trump administration wants to permanently change the regulatory environment for business, he said, the E.P.A. cannot take such a “quick and dirty approach” to unraveling regulations. “I’m suspicious that two, three years down the road there’s going to be much to show for all the fireworks we’re getting now,” Mr. Adler said.