Castle Mountains is surrounded on three sides by Mojave National Preserve. It is set to become one of three new national parks Friday. (David Lamfrom)

President Obama has set aside more of America’s lands and waters for conservation protection than any of his predecessors, and he is preparing to do even more before he leaves office next year. The result may be one of the most expansive environmental and historic-preservation legacies in presidential history.

On Friday, Obama designated more than 1.8 million acres of California desert for protection with the creation of three national monuments: Castle Mountains, Mojave Trails and Sand to Snow. The new monuments will connect three existing sites — Death Valley and Joshua Tree national parks and the Mojave National Preserve — to create the second-largest desert preserve in the world.

Namib-Naukluft National Park in Namibia is the largest.

Obama has unilaterally protected more than 260 million acres of America’s lands and waters under the Antiquities Act of 1906, which gives the president wide latitude to safeguard at-risk federal lands that have cultural, historic or scientific value.

The act is among the most powerful tools at any president’s disposal. Franklin D. Roosevelt invoked the law more than any president in history; Harold L. Ickes, his interior secretary, kept a pile of potential national-monument declarations in a desk and pulled them out whenever Roosevelt was in a good mood.

Obama’s aides do not have a similar system, but they share those earlier aspirations.

“We have big, big ambitions this year, so let’s see what happens,” said Christy Goldfuss, managing director of the White House Council on Environmental Quality, adding that the administration is focused on “local requests for action. It’s really been driven by activities on the ground.”

The big question: What next?

Other possible future designations include Bears Ears, a sacred site for several Native American tribes in southeastern Utah; Stonewall, the site of a 1969 inn riot by members of New York City’s gay community; the New England Coral Canyons and Seamounts; the historic headquarters of the National Woman’s Party, Sewall-Belmont House in Washington, D.C.; and Nevada’s Gold Butte, an area where rancher Cliven Bundy and his supporters have defied federal authorities.

Officials are weighing these proposals amid protests out West, such as the armed occupation of Oregon’s Malheur National Wildlife Refuge, which aimed to wrest control of federal lands from officials in Washington. The standoff may have hurt the prospects for increased protections around the state’s Owyhee Canyonlands, though the idea is not off the table entirely.

But Jim Messina, a close Obama adviser who worked on conservation issues when he served as White House deputy chief of staff in his first term, said the president is personally committed to the issue and is convinced that most Americans back the idea.

“Protecting public access is a huge political winner across the West. A bunch of extremists in Oregon can’t change it,” he said. “There’s no thought, or no reason, to back off on our agenda.”

Senate Minority Leader Harry M. Reid (D-Nev.), who convinced Obama to declare a sizeable monument in Nevada’s Basin and Range Province last year, is still pressing for getting another one at Gold Butte, which is an hour’s drive from Las Vegas but has been degraded and largely unpoliced since Bundy and his armed followers confronted Bureau of Land Management officials there in 2014.

Republicans have been trying to curtail Obama’s powers to act, but in a year when several senators are up for reelection in swing states, they have fallen short. Last week, the Senate considered an amendment by Sen. Mike Lee (R-Utah) that would have reversed national-monument designations if Congress and lawmakers in the affected states did not explicitly approve them within three years of designation. Four Republicans — including Kelly Ayotte (N.H.) and Mark Kirk (Ill.) — broke ranks and voted against it, and it was defeated 48 to 47.

House Natural Resources Committee Chairman Rob Bishop (R-Utah) said in an interview Wednesday that he was not surprised at the vote’s outcome. “

Most people do not understand what Antiquities does, or can do,” he said. “At some point, we have to realize this is a process that is out of control. Whether that actually occurs before Obama leaves is irrelevant.”

The Obama administration and Bishop have starkly different readings of the law, which runs just four paragraphs. It dictates that any monument designation “shall be confined to the smallest area compatible with proper care and management of the objects to be protected,” but presidents have interpreted that broadly over the past century.

The White House has identified two main criteria for naming monuments this year, Goldfuss said: areas that help foster resilience to climate change or are “connected to people and communities that have not been historically represented” in national parks and other federal sites.

That explains new California desert designations, for which Sen. Dianne Feinstein (D-Calif.) has been seeking protection for seven years. David Lamfrom, who directs the National Parks Conservation Association’s California desert and national wildlife programs, said connecting the ecosystem across nearly 10 million acres will help species with large ranges, such as bighorn sheep and mountain lions, as well as imperiled desert tortoises and ones that are taking refuge at higher altitudes where there is more moisture.

The idea is “to link together these large landscapes in perpetuity,” Lamfrom said, so species can migrate and have the best chance of survival in the face of human pressures.

Five members — the Navajo Nation, the Hopi Tribe, the Ute Indian Tribe, the Ute Mountain Ute Tribe and the Pueblo of Zuni — have created the Bears Ears Inter-Tribal Coalition to press for a monument on roughly 1.9 million acres in of Utah that were once inhabited by the Anasazi and, later, the Navajo.

Eric Descheenie, who co-chairs the coalition and serves as executive staff assistant to the Navajo Nation president, said: “We’ve had the looting and grave robbing and destruction of sacred sites,” even as several tribes have continued to gather medicinal herbs and berries, haul wood, hunt and conduct religious ceremonies there.

In some instances, Republican lawmakers have offered their own vision of how to protect these areas, but bipartisan agreements have proven elusive. Rep. Paul Cook (R-Calif.) has introduced a California desert bill that would put more than 1.2 million acres in the region off limits to development, but it would bar the use of the Antiquities Act, open up 100,000 acres of new mining in Mojave Trails and sanction off-road vehicle use in some areas.

It is less clear what Obama will do in federal waters, where nearly all of the strict protections are in the central Pacific. There are a group of Hawaiians lobbying the president to expand Papahanaumokuakea — a monument George W. Bush created a decade ago, whose islands and atolls are home to 1,750 marine species found nowhere else on Earth — to the full extent under the law. That would make it 520,000 square miles, or nine times its current size.

“Some people here are working here to provide the president with a legacy opportunity,” said William Aila Jr., looking down from a rocky outcropping in Oahu as two endangered Hawaiian monk seals nestled below. “It would be the largest marine protected area for a long, long time. It would be almost impossible to top it.”