The law says that presidents should limit designations to the “smallest area compatible with proper care and management.” In both cases, Utah politicians have argued that the actions of the previous presidents abused the law by exceeding that limit and were illegal.
Environmentalists and some native nations say Mr. Trump’s decision will destroy the national heritage and threaten some 100,000 sites of archaeological importance in the monuments’ desert landscapes.
Conservative lawmakers and many Westerners argue that the move is the proper response to decades of federal overreach that have sometimes starved communities of revenue and autonomy. When Mr. Clinton formed Grand Staircase, the move halted plans for a coal mining project there that would have brought desperately needed jobs to a poor county.
Mr. Trump’s move is viewed as a victory for Republican lawmakers, fossil fuel companies and others. The federal government controls about two-thirds of the land in Utah, and the state’s leading politicians have long pushed for more local control.
“He’s been sympathetic to the fact that we’ve been mistreated,” Senator Mike Lee of Utah said of the president, “and I’m grateful that he is willing to correct it.”
The announcement sparked an immediate outcry from Mr. Trump’s opponents. “What’s next, President Trump, the Grand Canyon?” said Rhea Suh, president of the Natural Resources Defense Council.
In April, Mr. Trump ordered the secretary of the interior, Ryan Zinke, to review 27 national monuments created since 1996, a process he said would “end another egregious use of government power.” In August, Mr. Zinke delivered a report to the president suggesting that Mr. Trump change the boundaries of several of those monuments.
The struggle over public lands has been fought most fiercely in the West, and Mr. Trump’s decision was anxiously anticipated for months. On Monday morning, hundreds of people gathered outside the State Capitol, some in cowboy hats, to protest Mr. Trump’s announcement.
Helaman Thor Hale and Andrea Hale, both Native Americans, brought their three sons to the rally. “It’s a historical trauma our people have been through over and over,” Mr. Hale said of Mr. Trump’s move.
Farther south, at the edge of the monument, another group gathered to applaud Mr. Trump’s decision standing beneath a banner: “Thank you for listening to local voices.”
The Navajo Nation, along with other tribes and conservation and outdoor industry groups, has vowed to challenge the decision to reduce both monuments in court, and several lawsuits are expected.
“We will stand and fight all the way,” said Russell Begaye, president of the Navajo Nation, adding that the United States government had already taken “millions of acres of my people’s land.”
At least one lawsuit was filed late Monday — by the Wilderness Society, Great Old Broads for Wilderness and eight other groups — in defense of Grand Staircase.