The League of Conservation Voters Action Fund’s endorsement of Hillary Rodham Clinton Monday has prompted a backlash from many of its members, who argue Sen. Bernie Sanders (I-Vt.) deserved the nod.

The endorsement, which was first reported by The Washington Post, marked the first time in more than three decades that the group had endorsed a presidential candidate before a single primary vote was cast. The group’s board Chairwoman Carol M. Browner, who served as the Environmental Protection Agency administrator under President Clinton and advised President Obama on climate change during his first term, said Hillary Clinton won the endorsement because she was best prepared to advance environmental priorities in office.

“You have to hit the ground running when it comes to tough issues like climate change,” Browner said. “Hillary Clinton gets what it takes to hit the ground running.”

Clinton said the endorsement of a prominent environmental group was not only “incredibly impactful” but would allow her “to start the process of being your partner as we build on the progress that has been made against pretty steep odds, and keep going here at home and around the world.”

“Because after all,” she told a crowd in Derry N.H. “I think we have to use every tool we have. There is no Planet B, this is it.”

Many Clinton stalwarts hailed the announcement: hundreds of people, including former Natural Resources Defense Council President Frances Beinecke, “liked” LCV’s Facebook post outlining its decision. But the move touched off a furor among Sanders supporters, who noted that he had a 95 percent lifetime rating from LCV compared to Clinton’s 82 percent. While the group does not evaluate governors, the third Democratic presidential contender, former Maryland governor Martin O’Malley, received an A-minus rating from LCV’s Maryland affiliate for 2007-2008 and a B-plus for 2009-2011.

Backers of Sanders posted a slew of comments on Facebook and Twitter, suggesting that the League of Conservation Voters’ federal political action committee had made a politically expedient choice. They noted that Clinton did not come out against the contentious Keystone XL pipeline, which Obama vetoed Friday, until September, while her rivals were early opponents.

Several vowed to withhold future donations to LCV in retaliation for the move and either give the money to other environmental groups, or Sanders himself.

“Bernie is obviously the better choice when it comes to environmental issues!!!!” posted Kimberlee Nelson. “He doesn’t have to think twice when it comes to protecting the environment unlike Hillary. ugh.”

[READ: Why LCV Action Fund picked Clinton over rivals with equal, or stronger environmental records]

Several environmental bloggers also questioned why a group focused on tackling climate would opt for Clinton over Sanders, who recently co-authored legislation to ban any future coal, oil or gas leasing from federal lands or waters. Brad Johnson tweeted, “Can be considered a climate hawk if she doesn’t support ?”

LCV’s political director Daniel J. Weiss replied that being able to appeal to the political center was as important as being an ideological purist.

Sanders campaign spokesman Michael Briggs, for his part, issued a statement saying his candidate’s “record on the environment is unbeatable.”

“That’s why he was endorsed by Friends of the Earth. That’s why Bill McKibben called him ‘the most aggressive voice in the Senate’ on climate issues,” Briggs added. “That’s why he has a 95 percent lifetime score from the League of Conservation Voters. The league agreed with former Sen. Clinton only 82 percent of the time, so its endorsement is based on something other than the merits.”

In an interview Monday evening, LCV Action Fund President Gene Karpinski said ” it’s not surprising that Senator Sanders’ supporters feel passionately” about their candidate.

“We continue to believe Hillary Clinton is best positioned to be an effective leader, and will be the best president on these issues from Day One,” he added.

In determining its endorsement, according to a statement, the political committee of the League of Conservation Voters’ board of directors reviewed questionnaires and conducted in-person interviews with each “pro-environment candidate” in the presidential race. The committee gave its recommendation to the full board of directors, which approved it.

The new endorsement got the attention of Republicans too: Republican National Committee spokesman Michael Short said it proved a Clinton presidency would undermine America’s economic viability.

“It’s clear Hillary Clinton plans to put ideology ahead of jobs, just like when she opposed the Keystone Pipeline,” Short said. “Too many families are being left behind in the Obama economy, and Hillary Clinton’s anti-energy agenda will only make it harder to raise wages and put more Americans back to work.”

Anne Gearan contributed to this report.