The Trump administration approved an oil leasing program for the Arctic National Wildlife Refuge Monday, opening up the pristine 19-million-acre wilderness to drilling for the first time and making it difficult to unwind the decision should Democrats recapture the White House in November.

Approving the program clears the way to auction oil leases “right around the end of the year,” Interior Secretary David Bernhardt said in an interview. The decision caps more than 30 years of efforts by oil companies and Alaskan leaders to drill in the refuge.

Environmentalists have raised concerns about the impact drilling could have on the polar bears and caribou herds that live in the remote refuge in northeast Alaska. Congress passed a mandate to lease oil from part of the refuge in its tax overhaul in 2017, when both the House and Senate were in Republican control. Mr. Bernhardt said the drilling can be conducted in an environmentally sound manner and that Congress has set details into law that will help the plan withstand challenges from environmentalists.

“Congress gave us a very clear directive here, and we have to carry out that directive consistent with the directive that they gave, and consistent with the procedural statutes,” Mr. Bernhardt said. “I have a remarkable degree of confidence that this can be done in a way that is responsible, sustainable and environmentally benign.”

The refuge, often known by its acronym ANWR, is nearly the size of South Carolina, nestled between the Arctic Ocean to the north and Canada’s Yukon to the east. Congress approved protections for it in 1980, and its expansive tundra, mountains and coastal plain are still nearly void of people and roads.