Mr. Bernhardt was narrowly confirmed to his current post by a vote of 53 to 43, with most Democrats voting against him. He is likely to face further scrutiny in his next confirmation hearings. If enough Democrats opposed his nomination, they could block a procedural motion requiring 60 votes to bring his confirmation to the Senate floor.
Mr. Bernhardt, who was also a top interior official in the George W. Bush administration, went on to work for some of the country’s largest oil and gas companies. As a partner in the law firm Brownstein Hyatt Farber Schreck, he lobbied for the oil companies Cobalt International Energy and Samson Resources. His legal clients have included the Independent Petroleum Association of America and Halliburton Energy Services, the oil- and gas-extraction firm once led by former Vice President Dick Cheney.
In August 2017, Mr. Bernhardt signed an ethics letter saying he would recuse himself from policy decisions that might stand to benefit former clients specifically.
If confirmed, Mr. Bernhardt will lead a sprawling department that oversees the nation’s nearly 500 million acres of public land, including vast national monuments and protected wilderness areas. Already, in little more than a year as the department’s deputy, he has overseen numerous polices aimed at opening public lands and waters to mining, drilling, farming and other development.
Environmentalists see him as a threat. “David Bernhardt is the most dangerous man in America for endangered species and public lands,” said Noah Greenwald, the endangered species director at the Center for Biological Diversity, an advocacy group, adding that he “has been dismantling basic protections for lands that belong to all of us and the vulnerable species, like the sage grouse, that depend on them.”
This year, Mr. Bernhardt oversaw the revision of a program to protect tens of millions of acres of habitat of the imperiled sage grouse, a puffy-chested, chickenlike bird that roams over 10 oil-rich Western states. His proposal to change that plan, made public in December, would strip protections from about nine million acres of the sage grouse habitat, a move that would open more land to oil and gas drilling than any other single policy action by the Trump administration.
Mr. Bernhardt has also helped shepherd policies such as loosening the standards of the Endangered Species Act, speeding the path to opening the Arctic National Wildlife Refuge to new oil and gas drilling, and reducing the boundaries of national monuments to open the land to mining and drilling.
Ethics inquiries into Mr. Zinke’s activities contributed to his departure in early January. He had become the subject of several federal investigations, one of which his department’s top watchdog has referred to the Justice Department, a potential step toward a criminal investigation.
The inquiries include an examination of a real estate deal involving Mr. Zinke’s family and a development group backed by David J. Lesar, the chairman of Halliburton. Mr. Zinke stood to benefit from the deal, while Mr. Lesar’s oil services company stood to benefit from Mr. Zinke’s decisions on fossil fuel production.
As public criticism of Mr. Zinke increased this fall with the news of the Halliburton deal and the Justice Department investigation, John F. Kelly, Mr. Trump’s chief of staff, sent Mr. Zinke a message: He should leave by year’s end or risk being fired in a potentially humiliating way, two people familiar with the discussion said.
In a statement posted on Twitter shortly after his resignation was announced, Mr. Zinke wrote: “After 30 years of public service, I cannot justify spending thousands of dollars defending myself and my family against false allegations. It is better for the President and Interior to focus on accomplishments rather than fictitious allegations.”
Among the first major decisions awaiting Mr. Bernhardt will be how to handle the administration’s plan to open the nation’s coastlines to offshore drilling — one of the issues for which Mr. Zinke is under investigation. After the Trump administration in early 2018 announced it would allow new offshore oil and gas drilling in nearly all United States coastal waters, Mr. Zinke made a surprise announcement a few days later on Twitter that he would exempt Florida from that plan.
The statement, which was accompanied by a photograph of Mr. Zinke and Rick Scott, the former Florida governor who was then running for a Senate seat, was seen as politically motivated. A federal investigation is continuing into whether it violated the Hatch Act, which prohibits federal employees from using their offices to influence elections.
Governors of other coastal states have said that they, too, would like to be exempt from the drilling, and that if the plan to exempt Florida but not other states goes forward, they will sue the department.