A major environmental group, the NRDC Action Fund, endorsed Hillary Clinton on Tuesday in its first political endorsement in a presidential election.

In a statement, the NRDC Action Fund, a political affiliate of the Natural Resources Defense Council, said the unprecedented endorsement reflects a need for left-leaning groups to unite against Donald Trump, the likely Republican nominee.

“Hillary Clinton is an environmental champion with the passion, experience and savvy to build on President Obama’s environmental legacy,” Rhea Suh, president of the NRDC Action Fund, said in a statement. “More than any other candidate running, Hillary Clinton understands the environmental challenges America faces, and her approach to solving them is grounded in the possibility and promise our democracy affords us.”

Suh specifically cited Trump's recent energy speech in North Dakota as one of the main rationales for the announcement.

"Donald Trump, on the other hand, has recently outlined a disastrous and frankly nonsensical environmental agenda — suggesting that he would tear up the Paris Climate Agreement, and that there is no drought in California," Suh said. "His plan for his first 100 days would take us back 100 years, and America cannot afford to indulge his climate conspiracy theories."

Clinton said she is "honored" to have received the group's backing in a statement on Tuesday.

"The stakes for our children's health and the future of our planet have never been higher," Clinton said. "We need to be accelerating the transition to a clean energy future, not letting a climate denier doom our children to a future beyond their capacity to repair."

The endorsement is a blow to Sen. Bernie Sanders of Vermont, who hopes to upset Clinton next week in the Democratic primary in California, a state where environmental issues loom large.

The NRDC Action Fund's move to endorse Clinton comes at a critical time for her as she seeks to dispatch Sanders and pivot to a general-election fight against Trump. Clinton may also be facing a tougher-than-expected challenge from Sanders in California. Her campaign announced Monday that she would spend more time than originally planned campaigning in the state this week.

It is the latest sign that Democrats and their allies are urging their supporters to unite behind Clinton, who is expected to clinch the Democratic nomination before polls close on the West Coast on June 7 after securing delegates in New Jersey's primary the same day.

“Hillary Clinton is all that stands between us and Donald Trump’s radical proposals to reverse decades of environmental progress,” NRDC Action Fund Board Chair Patricia Bauman said in a statement. “We’re making this historic endorsement because it’s critical for all of us to unite behind an environmental champion and help her beat Donald Trump.”

While both Sanders and Clinton largely agree on most environmental issues, he has accused her of being slow to oppose the Keystone XL pipeline, which is anathema to environmentalists. Sanders has also argued that the Paris climate agreement signed by President Obama in 2015 does not go far enough.