WASHINGTON — With Interior Secretary Ryan Zinke departing at the end of the year, the agency will likely be run, at least for a time, by its deputy secretary, David Bernhardt, a former oil lobbyist who has played a central role in enacting President Trump’s agenda of rolling back conservation measures and opening up public lands to drilling and mining.

Mr. Bernhardt’s supporters and detractors say that while Mr. Zinke has been the public face of some of the most significant reversals of public land protections in the nation’s history, behind the scenes it has been Mr. Bernhardt pulling the policy levers to enact Mr. Trump’s aggressive energy agenda.

“Bernhardt has been running the policy show ever since he’s been there as deputy secretary,” said Kathleen Sgamma, the president of the Western Energy Alliance, a Denver-based association of independent oil and gas companies.

It was Mr. Bernhardt, for example, who oversaw a controversial revision of a program to protect tens of millions of acres of habitat of the imperiled sage grouse, a puffy-chested, chicken-like bird that roams over 10 oil-rich Western states. Mr. Bernhardt’s proposal to change that plan, released just this month, would strip away protections from about nine million acres of the sage grouse habitat, a move that in a stroke opened up more land to oil and gas drilling than any other single policy action by the Trump administration.