Sen. Mark Begich issued a statement Monday opposing the development of Pebble Mine.
The announcement marks the first time the Democrat opposed development of the region.
His decision was influenced heavily by a study by the Environmental Protection Agency that suggests there is substantial risk to the region's salmon fishing industry, according to Begich.
The senator is also locked in a hotly-contested race to hold onto his seat: Lt. Gov. Mead Treadwell, Department of Natural Resources Commissioner Dan Sullivan and Joe Miller are Republican challengers for the seat.
Below are statements from pro- and anti-Pebble groups and some reactions from social media: