Bernie Sanders speaks at a campaign event at Drake University on June 12, 2015 in Iowa.National Journal

The first national green group to make an endorsement in the 2016 election just threw its weight behind Bernie Sanders.

Announced Saturday afternoon in New Hampshire, the endorsement from Friends of the Earth Action arrives amid rising frustration from the environmental Left over Hillary Clinton's refusal to take a stand on the Keystone XL pipeline and a host of other hot-button issues such as drilling in icy Arctic waters.

"We're seeing a lot of speeches from candidates, but Bernie has an incredibly strong track record and there's a lot of credibility there. He's also willing to say 'no' where some other candidates aren't," Erich Pica, the president of Friends of the Earth Action, said in an interview.

Many environmentalists fear that Clinton's cozy relationship with Wall Street and silence on some environmental issues may be a sign that she won't go far enough at a time when aggressive action is needed to stem the tide of global warming. And they see Sanders as a more-steadfast environmental champion.

Unlike Clinton, Sanders is on record saying that he supports a carbon tax, opposes Arctic offshore drilling, has voiced support for a ban on fracking in Vermont, supports the divestment movement, and opposes Keystone XL.

Clinton supports President Obama's effort to curb carbon emissions from power plants, a landmark policy aimed at tackling the greenhouse gas emissions that scientists say are driving dangerous global warming. She has promised to make climate change and the environment a key issue during her campaign, and started rolling out her climate agenda this week.

But some environmentalists say that won't cut it if Clinton wants their support.

"Regulating the poison isn't enough, we have to stop producing the poison, and Senator Sanders has been very clear about that. We have to say no to coal, no to oil drilling, no to gas drilling," Pica added.

Sanders already talks about the need to act on climate, but Friends of the Earth Action hopes that its support can push him to spotlight environmental issues even more. The group also hopes that Sanders's big-picture message of tackling income inequality will build momentum for the cause of environmental justice.

"Addressing global warming is one of the most important things that the next president can do, and the more that Senator Sanders can tie climate change to this larger economic message and message about corporate power, the more that helps to build this movement," Pica said.

Martin O'Malley, another Democratic 2016 candidate, also has outlined an ambitious climate agenda. That won O'Malley praise from environmental billionaire Tom Steyer. But while Sanders has drawn massive crowds, O'Malley is struggling to register in national polls, a factor that may add to some environmentalists' enthusiasm for Sanders.

Major environmental groups such as the Sierra Club, the League of Conservation Voters, and Steyer's NextGen Climate have offered up praise for Clinton, but haven't endorsed a 2016 candidate.

Friends of the Earth Action declined to say if it would spend money to support Sanders' 2016 bid and would not say if the group plans to make an endorsement during the general election if the Sanders campaign stalls out.

"Right now, Senator Sanders is helping to energize a progressive movement in this country and he's talking about issues that deeply need to be addressed. His candidacy and the way he's running it are giving permission to all the progressives out there to go out and publicly advocate on these issues. Ultimately, that will be a powerful message regardless of what happens in the presidential primaries and the election," Pica said.

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