In a statement, Mr. Zinke said monument designation was “not the best use of the land.” Mr. Zinke recommended that Mr. Trump roll back the boundaries to protect only areas that include historic and prehistoric structures, such as archaeological sites and remains of dwellings.

The monument, as it stands, is a vast canyon region of red rocks named for two towering buttes called the Bears Ears. It is home to some 100,000 archaeological sites.

The Antiquities Act, signed by Roosevelt in 1906, gives presidents the authority to protect designated areas as public monuments. In the century since its passage, presidents have used the law to protect millions of acres of public lands, including the Grand Canyon and the Muir Woods in California.

In recent years, however, conservative lawmakers have criticized Democratic presidents for what they view as overuse of the act. The move to protect Bears Ears was particularly controversial because of the monument’s size; it is more than 2,000 square miles, four times larger than Canyonlands National Park, the largest national park in Utah.

But no president has ever used his authority to eliminate or drastically reduce the size of a monument. (Presidents have on occasion modified monuments designated by their predecessors.)

Mr. Zinke declined to quantify the exact area of the proposed smaller monument, but said that the new boundaries should be limited to “the smallest area compatible” with the management of those sites.

“These recommendations were not made in a bubble in Washington, D.C.,” Mr. Zinke said in a phone call with reporters. “They were made after extensive on-the-ground consultation in Utah.”

Farmers, ranchers, and the oil and gas industry have urged the Utah congressional delegation to push for a rollback of the protected areas so they could have access to the land for development.

But opponents of the boundary reductions said that Mr. Zinke’s description of the area that would still be eligible for protection appeared to account only for a small fraction of the acres designated by Mr. Obama.

“If you look at a map, that area is only about 5 percent of the monument area,” said Adam Sarvana, a spokesman for Democrats on the House Natural Resources Committee.

“It seems like what they’re describing is a few stops on a boardwalk arcade, a few isolated areas, rather than a professionally conserved landscape the way national monuments are typically designated,” Mr. Sarvana said.

“Make no mistake: Unilaterally shrinking the boundaries of Bears Ears National Monument would not only be a slap in the face to the five sovereign tribes who share sacred ties to this land, it would violate both the Antiquities Act and the separation of powers doctrine,” said Heidi McIntosh, a lawyer for the advocacy group Earthjustice. “The president simply lacks the authority to change a national monument designation under the Antiquities Act, our country’s century-old law that protects some of our most scenic and historic landscapes.”

Mr. Zinke also recommended that Congress pass legislation to designate some areas within the existing monument as national recreation areas or conservation areas, which have different levels of protection.

The monument has emerged as a lightning rod in the fight over control of land in the West. Mr. Obama set it aside on Dec. 28. But in late April, Mr. Trump signed an executive order directing Mr. Zinke to review the boundaries of 27 areas protected under the Antiquities Act.

Mr. Zinke’s recommendation to scale back the boundaries of Bears Ears does not represent the administration’s final action on the matter, but it does appear to signal that the Trump administration intends to explore the idea of reducing the size of other protected areas.

“It’s a victory,” Hanson Perkins, 32, who lives near Bears Ears National Monument, said of the announcement. He is hoping to drill for oil in the area. “There is enough local outrage about it, they could rescind it and be within reason.”