But Bernie Sanders campaign cried foul, questioning the validity of the endorsement given the fact Sanders' voting score from the same group is higher than the one they give Clinton.
"We think the stakes are so high, there is so much at stake, we need to get in now to make sure Hillary wins," Gene Karpinski, head of the environmentalist group, said Monday to explain why the group was endorsing earlier than they ever had before. "That is why we are here. We have got to win."
Karpinski's comment on winning speaks to his group's history of endorsing the candidate more likely to win, as opposed to those most ideologically in line with their group. The environmentalist group, for example, endorsed Maine Republican Sens. Olympia Snowe
in 2006 and Susan Collins in 2014
, touting their bipartisan leadership.
After the event, Karpinski said it was Clinton's "experience" and "leadership" that separated her from the rest of the field, but declined to answer to Sanders' complaints or whether they backed Clinton because she was the front-runner.
"All three Democratic candidates have strong records," he said. "We endorsed Hillary Clinton today, as we made clear, because we believe she will be the most effective person to get the job done from Day One."
But after the League of Conservation Voters Action Fund announced they would endorse Clinton on Monday morning, Sanders' campaign cried foul.
"Bernie's record on the environment is unbeatable," said Michael Briggs, Sanders' spokesman, before listing the senator's record on the environment. "He has a 95% lifetime score from the League of Conservation Voters. The league agreed with former Sen. Clinton only 82% of the time, so its endorsement is based on something other than the merits."
Clinton stayed above the back-and-forth on Monday when she accepted the endorsement, instead focusing on her environmental platform.
"Having this endorsement means so much to me personally," Clinton said. "I believe strongly that we can make the case, we can convert some of the skeptics, but it will only happen if we are going to be stalwart in our focus on this issue in being central not only to this campaign but for everything we do politically going forward."
The former secretary of state also addressed the forthcoming 2015 Paris Climate Conference, where representatives from nations across the country will come together to try to hammer out an international deal addressing climate change.
Clinton said she had "high hopes and expectations that there can be a new, ambitious, lasting climate agreement to be accepted" and took a little credit, arguing that if a deal is struck "it will be built on the foundation that the President and I laid in Copenhagen."
After the event, CNN asked Clinton if she truly believed a deal could be struck, something even optimistic environmentalists see as a long shot.
"I'm very hopeful," Clinton said. "I'm hopeful."