WASHINGTON — Donald Trump Jr., President Trump’s eldest son, took a break this week from his relentless Twitter attacks on Democrats to express his concern toward a different target: his father’s administration.
The younger Mr. Trump and Nick Ayers, the former chief of staff to Vice President Mike Pence, tweeted on Tuesday their opposition to final government approval of the Pebble Mine, a vast gold and copper mine to be dug near salmon fisheries not far from the pristine Bristol Bay in Alaska.
The two men said they hoped the president would block development of the mine by a Canadian company in the interest of protecting the area’s sensitive environment.
“This should be stopped and I believe @POTUS will do so!” Mr. Ayers wrote.
Left unsaid was that the mine’s opening was set in motion three years ago by the president’s business-friendly administration, which has pushed the project forward ever since.
“As a sportsman who has spent plenty of time in the area I agree 100%” with Mr. Ayers, the younger Mr. Trump tweeted to his 5.4 million followers. “The headwaters of Bristol Bay and the surrounding fishery are too unique and fragile to take any chances with. #PebbleMine.”
The stance from the president’s son and a former top official is a rare and curious instance of public disagreement, especially from Donald Trump Jr., who is the very definition of a loyal warrior for his father’s presidency.
Under President Trump, the Environmental Protection Agency reversed an Obama-era decision to block the project, allowing an environmental review by the United States Army Corps of Engineers to proceed. The final version of the review, released last month, found that the project would not result in “long-term changes in the health of the commercial fisheries in Bristol Bay.” A decision to grant a permit to the project is expected in about a month.
The Pebble Mine, about 200 miles from Anchorage, would be one of the largest open-pit mines in the world, with copper, gold and other metals potentially worth hundreds of billions of dollars. But it would sit in salmon breeding grounds that are the basis for a large sport-fishing industry, as well as a commercial fishery in Bristol Bay that employs about 15,000 people.
The younger Mr. Trump is a well-known hunter and sportsman, though it is not clear exactly why he chose to voice his opposition to this particular project. Environmentalists have criticized many other decisions by the president on which his son has been silent.
A person familiar with the thinking of the president’s son said he had spent time in Bristol Bay and believed that digging the mine near the sensitive fisheries could result in serious damage if there were an accident involving potentially toxic tailings at the mine. It was unclear whether he had mentioned his concerns to the president recently, but the person said Donald Trump Jr. had raised it several times since the 2016 election.
President Trump, asked Wednesday evening about his son’s tweet, was noncommittal but said, “I will look at both sides of it.”
Nanci Morris Lyon, a fishing guide and the co-owner of Bear Trail Lodge on the Naknek River near Bristol Bay, hosted Donald Trump Jr., his son Donald III and his brother Eric Trump in July 2014 for Eric Trump’s bachelor party.
Ms. Lyon, who said she had been an outspoken opponent of the mine for more than a decade, spent a lot of time with the Trumps over 10 days. “I’ll guarantee you he heard about Pebble Mine,” she said, referring to Donald Trump Jr. “Because if you go fishing with me, you’re going to hear about it.”
Ms. Lyon said that Donald Trump Jr. and his brother were “very grateful” to have had the experience of fishing in such an untouched, wild place. “They truly got it,” she said. “They understood how ludicrous this expansive mine was.”
Mr. Ayers did not respond to questions about why he opposed the mine. But in his tweet, he claimed common cause with “millions of conservationists and sportsmen” and said that allowing the Pebble Mine project to go forward would “unnecessarily mine the USA’s greatest fishery at a severe cost.”
The White House declined to comment, referring questions to the Corps of Engineers, which said in a statement that “it is inappropriate for us to comment on opinions, to speculate on potential outcomes of our deliberations in response to media inquiries or to answer technical questions.”
Mike Heatwole, a spokesman for the Pebble Partnership, the company developing the mine, said the recently completed environmental review “shows that both Donald Trump Jr. and Nick Ayers are wrong.” The review, he noted, “concludes that Pebble Mine will not harm the Bristol Bay salmon fishery.”
He added, “We do not believe that the president will interfere with this statutory process.”
The project has been a subject of intense debate in Alaska for nearly two decades, pitting mining interests against fishing interests and environmentalists against those in favor of exploiting the state’s resources.
Alaska Natives in that part of the state, where salmon has been a major subsistence food for centuries, are divided. Many oppose the mine because of the threat to salmon, but some villages and groups are in favor because of the potential for jobs and infrastructure improvements in the remote region.
Chris Wood, the president and chief executive of the conservation group Trout Unlimited, which opposes the mine, said he was happy to have Donald Trump Jr. — who he said was a member of his organization — publicly join the fight.
“If President Trump chooses to use his authority to stop this mine,” Mr. Wood said, “he will be a hero to millions of hunters and anglers.”