Updated at 9:42 p.m. ET

After a pressure campaign by people with President Trump's ear, the Army Corps of Engineers says a proposed open-pit gold mine upstream from Alaska's most valuable salmon fishery can't proceed as the developers hoped. It's not an outright veto of the mine, but some of the groups fighting the project say the years-long battle is as good as won.

Until now, the Pebble Mine seemed on track to get its federal permit as early as this fall, to the alarm of fishermen and Alaska Native organizations.

They say the mine is an existential threat to the way of life in Bristol Bay, where the economy and the culture revolve around commercial and subsistence salmon fishing. This part of southwest Alaska is also renowned for expensive sport-fishing lodges.

Mine supporters say the project would bring year-round jobs to an area that badly needs them and provide minerals the nation needs. The Pebble Partnership has run ads to highlight a conclusion the Corps of Engineers reached this summer: that the mine would have "no measurable impact" on Bristol Bay's salmon population or fishery.

But shortly after that conclusion several influential Republicans spoke out publicly against the mine. They include the president's son Donald Trump Jr., who took his son on a fishing trip to Bristol Bay and has the Facebook photos to prove it.

"As a sportsman who has spent plenty of time in the area I agree 100%," Trump Jr. wrote on Twitter on Aug. 4 in response to an anti-Pebble tweet from another Trump administration insider, Nick Ayers. "The headwaters of Bristol Bay and the surrounding fishery are too unique and fragile to take any chances with. #PebbleMine."

Bass Pro Shops founder Johnny Morris then aired his argument against the mine on Tucker Carlson's Fox News show.

The president said he'd "listen to both sides."

"I've done a lot for Alaska. I love Alaska. It's a special place," Trump said at a press conference Aug. 5.

It's not clear the campaign had an impact on the president, but Pebble'ssuccess is suddenly in doubt.

Alaska's U.S. senators issued a statement praising the Corps' decision. They said the administration has determined Pebble had not met the high standards required, and the senators spoke of it with finality.

"I agree that a permit should not be issued," Sen. Lisa Murkowski said in the written statement.

And Sen. Dan Sullivan's remarks were much the same: "I support this conclusion ... that a federal permit cannot be issued."

While the Army Corps has not outright killed the mine proposal, it may have set conditions that are impossible to meet.

The issue is "mitigation," a feature of the Clean Water Act that says if you want to build a project that damages wetlands or streams, you must offset that damage with improvements, ideally in the same place that's harmed.

The mine and its transportation corridor would damage nearly 200 miles of streams and thousands of acres of wetlands. Pebble had planned to go farther afield for mitigation, to improve village sewage systems and replace culverts in Dillingham, the largest community on Bristol Bay.

The Corps now says that won't do.

In a letter dated Aug. 20 but released Monday, the agency mandated mitigation for "all direct and indirect impact" within the Koktuli River Watershed.

Pebble, owned by a Canadian firm called Northern Dynasty,insiststhis isn't the end of the line for the project. CEO Tom Collier says he expects to submit a mitigation plan in a few weeks.

Collier is just putting on a brave face, says attorney Joel Reynolds, Western director for the Natural Resources Defense Council. He says the Corps is asking for the impossible, because the Koktuli watershed has no damage on the scale that Pebble wants to inflict, so there's nothing for the company to mitigate.

"It's a perfectly functioning pristine, natural ecosystem," Reynolds said. "It's an incubator for trillions of salmon over thousands of years, and the notion that Northern Dynasty has anything to offer it in terms of improvement is ridiculous."

There's been heated debate over Pebble Mine for years. The Obama administration aimed to block it.

Other mine opponents say they won't rest easy until they see a rejection of the permit. They want the Environmental Protection Agency to exercise the veto power it has under the Clean Water Act. As long as Pebble is still trying to dig a mine at the headwaters of Bristol Bay, they say, their fight continues.

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