Democratic candidates Hillary Rodham Clinton, left, former Maryland Gov. Martin O'Malley, center, and Sen. Bernie Sanders, I-Vt, right, pose for a photo after a democratic presidential candidate forum Friday. (AP Photo/Chuck Burton) (Chuck Burton/AP)

The League of Conservation Voters Action Fund on Monday intends to endorse Hillary Rodham Clinton, the first time in three decades it has endorsed a presidential candidate before a single primary vote has been cast.

The decision to back Clinton over two Democratic rivals with equally strong, if not stronger, liberal environmental records shows the extent to which some environmentalists are concerned the Obama administration’s policies could be rolled back under a Republican president. The group’s president, Gene Karpin­ski, said it needs to activate volunteers and donors early to make sure Clinton is strongly positioned for the general election.

The LCV Action Fund picked Clinton over Sen. Bernie Sanders (Vt.) and former Maryland governor Martin O’Malley because “the stakes are so high” and Clinton “has proved she’s an effective leader who can stand up to the big polluters and push forward an aggressive plan to tackle climate change,” Karpin­ski said.

Karpinski and Carol M. Browner — the group’s board chair, who served as a climate-change adviser to President Obama and as Environmental Protection Agency administrator under President Bill Clinton — will make the announcement with Hillary Clinton in Nashua, N.H.

Until now, the earliest the LCV Action Fund, the federal political action committee affiliated with the League of Conservation Voters, had ever endorsed a candidate was when it backed then-Sen. John F. Kerry (D-Mass.) in January 2004. The LCV is a major campaign player among environmental organizations: The group and its related entities spent more than $30 million in the 2014 election cycle.

The LCV scores all federal lawmakers on their voting records, and Clinton has an 82 percent lifetime record, compared with Sanders’s 95 percent. While the group does not evaluate governors, its state affiliate Maryland LCV issues occasional governor scorecards, and O’Malley received an A-minus rating for 2007-2008 and a B-plus for 2009-2011.

LCV spokesman David Willett noted that the group counts any missed vote as a “no” vote and that votes Clinton missed while running for president lowered her overall rating.

All three Democrats have made climate change a central theme in their campaigns. As governor, O’Malley established a commission to respond to climate change, pledged to cut Maryland’s greenhouse-gas emissions 25 percent by 2020 and expanded renewable-energy development. Sanders introduced legislation to establish a carbon tax with Sen. Barbara Boxer (D-Calif.) in 2013, helped secure $3.2 billion in energy-efficiency grants as part of the 2009 stimulus bill, and recently co-authored a bill that would ban any future coal, oil or natural gas leases on federal lands and waters.

“The scientific community is telling us that if we do not address the global crisis of climate change, transform our energy system away from fossil fuel to sustainable energy, the planet that we’re going to be leaving our kids and our grandchildren may well not be habitable,” Sanders told CNN host Anderson Cooper during the first Democratic presidential debate when asked to identify the country’s greatest national security threat. “That is a major crisis.”

Clinton, for her part, has moved to the left on environmental issues over the course of her public career. In 2006, she voted for the Gulf of Mexico Energy Security Act, which expanded offshore drilling in the gulf; she recently announced her opposition to offshore drilling in the Arctic Ocean, as well as to the Keystone XL pipeline, which Obama blocked on Friday. Sanders and O’Malley had previously come out against the pipeline.

As secretary of state, Clinton helped negotiate a nonbinding climate agreement in Copenhagen six years ago and worked to phase out polluting cookstoves in developing countries. She recently released an energy plan that pledges the United States will generate enough renewable energy in 2027 to provide roughly a third of the nation’s overall supply, and that by the end of her first term there would be more than 500 million solar panels installed across the country.

The Clinton campaign did not respond to a request for comment on the upcoming endorsement.

Even as the Obama administration has elevated environmental issues and the Democrats seeking to replace him have pledged to go further, however, members of the GOP presidential field have stiffened their opposition to those policies. Every Republican presidential candidate except for Sen. Lindsey O. Graham (S.C.), who was recently cut from the GOP’s undercard debate because he is polling under 2 percent, has questioned the need for mandatory limits on greenhouse-gas emissions linked to climate change.

Obama’s denial of a presidential permit to TransCanada to build Keystone XL, for example, brought an immediate rebuke from two Republican candidates, Sens. Ted Cruz (Tex.) and Marco Rubio (Fla.).

“When I’m president,” Rubio said, “Keystone will be approved, and President Obama’s backward energy policies will come to an end.”