SALT LAKE CITY — Native American tribes and environmental groups preparing for a legal battle to stop President Donald Trump from dismantling Utah's new national monument face a tougher challenge than anticipated.

Republican officials in the state who oppose Bears Ears National Monument asked Trump to rescind the designation. But Interior Secretary Ryan Zinke recommended the monument be downsized instead, noting that past presidents have tinkered with the boundaries of lands protected under federal law.

Legal experts disagree on whether the 1906 Antiquities Act allows a president to reduce a monument, and it's something that has never been challenged in court.

Environmentalists and Native American tribes were ready to pounce at the notion Zinke would recommend Bears Ears be abolished, armed with their belief that no president may undo the work of another by rescinding a monument, and the fact that no president has tried.

But past presidents have trimmed national monuments and redrawn their boundaries 18 times, according to the National Park Service.

Bears Ears, established by President Barack Obama in December, is about the size of Delaware, covering roughly 2,000 square miles. It protects more than 100,000 archaeological sites on what's considered sacred tribal land in southeastern Utah.

A largely GOP group of Utah officials wants the monument repealed, seeing it as an overly broad, unnecessary layer of federal control that closes off the area to energy development and other access.

Rep. Mike Noel, R-Kanab, said shrinking a monument is politically and legally much easier to defend than attempting to undo one.

"There's been enough history of downsizing, even fairly large areas, significantly large areas," Noel said.

Many times, past presidents reduced monuments only slightly, like when Franklin Roosevelt removed about 52 acres from Arizona's Wupatki National Monument in 1941 to make way for a dam.

Occasionally the changes were drastic, like President Woodrow Wilson's move in 1915 to cut Mount Olympus National Monument in Washington state roughly in half to open more land for logging.

Environmental groups and others gearing up for a fight note that no president has tried to downsize a monument since the 1976 Federal Land Policy and Management Act, which they say restricts a president's ability to do so. The groups also contend that past presidents never faced court challenges for shrinking monuments.

"Whatever this administration does will certainly not go unchallenged," said Kristen Boyles, an attorney with the environmental group Earthjustice.

Legal experts disagree on whether the environmental groups are right, but the court battle that's expected if Trump tries to cut down Bears Ears could significantly alter what's generally been a lasting protection from presidents.

The 1906 Antiquities Act that gives presidents the power to declare monuments does not explicitly say whether a president can nullify a monument proclamation or shrink its boundaries.

Donald J. Kochan, a professor of natural resources, property and administrative law at Chapman University in Orange, California, said the president's broad power to create a monument comes with an inherent ability to change a monument or undo it, just as presidents regularly undo other policies or regulations from past administrations.

Mark Squillace, professor of natural resources law at the University of Colorado-Boulder, disagreed. Congress controls public lands, he said, and it's significant that in passing the Antiquities Act, lawmakers spelled out only that the president can create a monument.

Congress took care in other laws passed around that time, more than a century ago, to explicitly give the president powers to both act and undo acts, Squillace noted.

The 1976 land policy law and congressional records of the law's drafting also make it clear that Congress didn't want to give presidents the authority to shrink or undo monuments, he said.

The question about whether the president has the power to shrink a monument "is one of these big, lingering issues that's been out there for a long time," Squillace said. "I think there's a very strong case against the president's authority to do this."

Lawsuits are expected from the Navajo Nation, groups such as the Wilderness Society and Earthjustice, and even outdoor gear company Patagonia once Trump takes action on Bears Ears. That's not likely to happen until at least August, when Zinke finishes the president's request that he review 26 other monuments.

Noel said he's working on legislation that will commit the state to intervening in the lawsuit to help defend the Trump administration's action.

Representatives for Gov. Gary Herbert and Utah Attorney General Sean Reyes, both Republicans, declined to say whether they'd join a lawsuit.

Messages seeking comment from the Interior Department were not returned.