WASHINGTON — The Obama administration is preparing to designate areas in New Mexico and California off-limits to development under its executive authority, according to individuals familiar with the matter, a move that signals a bolder public-lands policy in the president’s second term.

The individuals, who asked not to be identified because a final decision has not been made, said that the White House is poised to act unless Congress moves soon on legislation that will afford similar protections.

One of the two sites, the nearly 500,000-acre Organ Mountains-Desert Peaks region near Las Cruces, N.M., is twice as large as the largest national monument established by President Barack Obama. The other site is about 1,600 acres on California’s central coast known as the Point Arena-Stornetta Public Lands.

Although Congress traditionally designates protection for public lands, presidents have used their authority under the 1906 Antiquities Act to set aside prized areas.

Obama drew an enthusiastic response from Democrats and conservationists when he said in his State of the Union speech that he would use his authority “to protect more of our pristine federal lands for future generations.”

The monument designations would address a longtime criticism among conservationists that the president has not done enough to safeguard ecologically sensitive and historically important federal lands. They represent one of the most powerful ways he can use his executive powers to achieve policies opposed by Congress.

But such a move is sure to anger some constituencies that rely on public lands for uses such as ranching, mining and motorized-vehicle recreation. They see any new protections as an encroachment on those activities.

The president declared five national monuments last spring, the largest encompassing 242,500 acres.

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