More than 40 U.S. House and Senate members have asked President Donald Trump to maintain protections for Alaska's Bristol Bay region, which produces about half of the world's sockeye salmon.
The Democrats say a federal proposal to ease restrictions on mining in the area is egregious and illogical.
The stalled Pebble Mine project received new life when the U.S. Environmental Protection Agency said it would move to lift restrictions on development sought by the Obama administration as part of a legal settlement.
The mine's developer recently unveiled plans for a smaller project.
The letter, spearheaded by Sen. Maria Cantwell of Washington and Rep. Jared Huffman of California, seeks additional public hearings and comment time.
Members of Alaska's all-Republican delegation didn't sign it.