Story highlights The League of Conservation Voters Action Fund, a key environmentalist organization, endorsed Hillary Clinton

But Bernie Sanders campaign cried foul, questioning the validity of the endorsement

Nashua, New Hampshire (CNN) The League of Conservation Voters Action Fund, a key environmentalist organization, endorsed Hillary Clinton on Monday in New Hampshire, touting her as the most qualified and prepared candidate in the field.

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.

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