PORTLAND — The Sierra Club has endorsed Maine independent Angus King in the race to fill Republican Sen. Olympia Snowe’s soon-to-be-empty seat, leaving Democrat Cynthia Dill disappointed.

Becky Bartovics from Sierra Club Maine said Monday that King knows that “investing in a clean-energy future will create new jobs in Maine and across the U.S., and protect our planet for generations to come.”

As governor, King backed the Land for Maine’s Future program to ensure open spaces. After leaving office, King launched a wind power venture. In its endorsement, the Sierra Club cited King’s support for clean energy policies and investments to transition the country to cleaner energy.

While Dill was disappointed, she brushed off the endorsement. She said she’s always been an environmentalist and doesn’t need an endorsement to verify that.

King is among 14 Democratic U.S. Senate candidates and one other independent who have been endorsed by the Sierra Club. Also endorsed by the organization is Vermont Sen. Bernie Sanders, who caucuses with Democrats but describes himself as a Democratic Socialist.

In a release issued by her campaign Dill said she was “disheartened” by the Sierra Club’s “inexplicable decision.”

“Despite its error, and despite the gender bias that so many national special interests have regarding this race, I promise Maine voters to remain true to who I am,” Dill said in the prepared statement. “In the final analysis, I don’t need environmental endorsements to verify that I am an environmentalist. I always have been.”

The campaign of Republican candidate and Maine Secretary of State Charlie Summers also reacted to the announcement Monday challenging King’s record on emissions testing for automobiles and his involvement in the wind energy industry in Maine.

“Between repealing the auto emissions testing program as governor and an evolving explanation of his role in blasting Maine’s mountaintops to make room for giant industrial windmills, Angus King’s record on energy and environmental stewardship is anything but clean,” Summers’ campaign spokesman, Drew Brandewie, said in a prepared statement.