Environmental group 350 Action is backing Kevin de León in his primary challenge to Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.) from the left.

The activist group and its co-founder, Bill McKibben, said de León, a current member and former leader of California's Senate, has shown a strong commitment to key issues like clean energy, environmental justice and fighting polluters.

“Kevin de León has been a strong champion of clean energy—and an effective one, using his power in Sacramento to make change happen against the strong opposition of the fossil fuel industry,” McKibben. “He won’t just vote the right way in DC, he will make the climate justice debate come alive!”

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May Boeve, 350 Action’s executive director, said de León “is rising to the challenge to make California a true leader on climate that goes above and beyond what we’ve seen nationally, and not a moment too soon.”

In backing de León, 350 Action joins Climate Hawks Vote — which endorsed him last year shortly after he announced his candidacy — and billionaire activist Tom Steyer.

Democrats closer to the mainstream, like former President Obama, former Vice President Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE and California Gov. Jerry Brown, have stuck with Feinstein.

De León has consistently presented himself as more progressive than Feinstein in areas including climate change.

Californians will cast their first votes in the nonpartisan primary next week. The two top finishers will then go head-to-head in the November election.

De León has consistently polled far behind Feinstein. A University of Southern California Dornsife–Los Angeles Times poll released last week had Feinstein with 31 percent of voters’ support and de León with just 7 percent.