The heart of the Antiquities Act of 1906 is a mere two sentences. But a good argument can be made that this brief law — which authorizes the president to protect “objects of historic or scientific interest” on federal lands as “national monuments” — has done more than any other to shape our nation’s conservation legacy.

The act has been used more than 150 times, by nearly every president, Republican and Democrat, from Theodore Roosevelt on, to protect hundreds of millions of acres for the inspiration and enjoyment of present and future generations. Five of the nation’s 10 most-visited national parks — Grand Canyon, Zion, Olympic, Teton and Acadia, each attracting millions of people a year — were first protected by presidents using the Antiquities Act.

Even so, this law is under attack. The 2016 Republican Party platform called for amending it to give Congress and states the right to block the president from declaring national monuments. By thwarting the president’s ability to take quick action to protect wild and historic places from threats, this proposal would effectively repeal the act.

Now critics, including Representative Rob Bishop, a Republican from Utah and chairman of the House Committee on Natural Resources, are ramping up a campaign to strip away the president’s authority under the Antiquities Act to designate monuments. Mr. Bishop complains that it allows the federal government to “invade” and “seize” lands. But that’s not true. The act authorizes the president to protect only lands already “owned or controlled by the government of the United States,” not state or private land.