Senator Bernie Sanders of Vermont announced Thursday that he's challenging Hillary Clinton for the Democratic nomination. Sanders' decision—as a vocal Keystone XL opponent, clean energy champion, and carbon tax bill sponsor—is good news for environmentalists who remain unconvinced Clinton will champion those issues. Just in January, he and other Democrats introduced amendments that forced Republicans to admit their climate change denial. Though he is a long-shot for the nomination, Sanders may prod Clinton into giving voters more detail on her environmental positions.

And yet, some environmentalists remain uninspired by Sanders. They're holding out for Massachusetts Senator Elizabeth Warren (who isn't running).

This week, Ready for Warren, a group trying to recruit Warren to run for president, announced the launch of Environmentalists for Warren. Ready for Warren organizer Erica Sagrans says more than 1,500 environmental supporters are involved, representing more than three dozen green groups (none of the organizations have officially endorsed any candidate).

These supporters sounded sure only Warren can do the job of pushing Clinton on the environment. 

"I love Bernie Sanders, but I think that Warren would represent greater change in a lot of ways. We're also thinking about what's going to catch fire more and Elizabeth Warren is just a rock star," Anthony Rogers-Wright, policy director for the nonprofit Environmental Action, told National Journal

On a Monday press call, a Vermonter lamented what she saw as environmental damage from industrial-scale wind turbine projects on mountaintops, and called out Sanders for siding with wind companies in the past. “Bernie Sanders—although he is so magnificent on the effect of corporate issues, the oligarchy, and all the rest of it—he is for industrial-scale wind turbines," she said. "He is also horribly uninformed, so we really need someone who really gets it. We really do. And I think Elizabeth Warren is the one." (Warren apparently hasn't taken a position on this specific issue.)

Warren may be a strong voice on economic inequality, but she has a relatively thin record on climate since she joined the Senate in 2013. To highlight Warren's bright record, Environmentalists for Warren mentioned only that she recently joined the Senate Committee on Energy and Natural Resources, has spoken out against Keystone, and is now rallying against the Obama administration’s trade deal, which environmentalists vehemently oppose. That's it.

R.L. Miller, founder of the new climate super PAC Climate Hawks Vote, scores candidates based on their climate change records. According to the Senate scoring, Warren earned 48 (out of 100) in the last Congress—a "middle of the pack" score, says the group: "Some Senate observers have expected Warren to discuss climate change more this year now that she is on the Energy & Natural Resources Committee, but so far she's only made one public statement, engaged in minimal press release work, not written any bills, and co-sponsored very few bills.”

Warren is a relatively new addition to the Senate, so supporters hope she will become more proactive on climate change over time. 

With Sanders, however, there is no guesswork. He scored 95 from Climate Hawks—the top score in the Senate. “So far, Sen. Sanders has shown substantially stronger leadership on climate than Sen. Warren has," Miller said. "He's introduced strong carbon-pricing and solar bills, marched with New Yorkers last fall, worked to bring solar technology to his home state, and been a loud and consistent voice in opposition to polluters.”

To be sure, Warren and Sanders are difficult to tell apart in many policy areas, and looking only at environmental votes, they're very similar. The League of Conservation Voters has scored Sanders and Warren's career votes at 95 percent and 94 percent, respectively. Where they differ is on public advocacy. In that arena, Warren simply can't compete with Sanders.