NEW YORK – Some of the prominent environmental activists who gathered Sunday in New York City for a massive climate change march are not ready to support Hillary Clinton if she decides to run for president.

“I think Hillary Clinton has an awful lot to demonstrate to environmentalists and people who care about climate change,” Bill McKibben, the founder of 350.org, who helped organize the march, told msnbc. “She oversaw the complete fiasco that was the Copenhagen Conference as secretary of state. That was the biggest foreign policy failure since Munich. It’s not a proud record.”

“I think Hillary Clinton has an awful lot to demonstrate to environmentalists …” Bill McKibben, founder of 350.org

And on Sunday, Clinton allies defended her environmental bona fides. “As Secretary Of State, Hillary Clinton led efforts to combat the growing threat of climate change both at home and abroad. Clinton created a special envoy for climate change, and she launched the Climate and Clean Air Coalition – a group of 37 countries that agreed to work to reduce their emissions. At the Clean Energy Summit held recently in Nevada, Clinton said climate change is ‘the most consequential, urgent, sweeping collection of challenges we face as a nation and a world,’” said Adrienne Watson of the pro-Clinton rapid response group Correct the Record.

But Van Jones, a former Obama White House advisor on green jobs, said Clinton is “going to have to reassure a lot of people because of her position on fracking,” the new technology used to extract oil and gas from shale. “I think she has some distance to make up in terms of reassuring people about where she comes down as a climate champion,” Jones told msnbc.

The demonstration in Midtown Manhattan, which drew tens of thousands of demonstrators of all ages and from around the world, attracted a number of celebrities and big name advocates, as well.

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“If someone can offer up another way forward, [climate voters] are going to get behind that person. Just like Obama, they got behind him,” Ruffalo continued, suggesting Sens. Elizabeth Warren or Bernie Sanders as possible alternatives. “These are people who get this, who are not beholden to this 100-year-old fossil fuel stranglehold on leaders like Hillary Clinton.”

A recent investigation from Mother Jones magazine detailed how Clinton’s state department promoted the fracking aboard and Maura Cowley, the Executive Director of the Energy Action Coalition, which organizes youth around environmental issues, also cited fracking as a concern.

“I think she’s done amazing work and made some bold commitments on climate change, but her support of fracking has been very troubling for the youth vote,” she told msnbc. “We’re really hoping for her to also take a bolder stance on tar sands. We’re very excited, but we think she needs to be doing more.”

“Unless she changes that, the climate change voters, the people who take this very seriously, are not going to be able to get behind her.” Mark Ruffalo

Tom Steyer, the billionaire Democratic donor and clean energy activist, supported Clinton in 2008 over Obama during the Democratic primary. At the march on Sunday, he told msnbc that he thought Clinton would make “a great president” and that he’s looking closely to see where she comes down on some key energy issues.

“I don’t think she has fully formulated what she’s going to say,” he said. “Obviously, for us to address this as a global problem, the chief executive of the United States need to lead, and so in order to do that you have to prioritize it and want to lead on it … It will be interesting to see.”

Earlier this month, Clinton gave a high-profile speech on energy in Las Vegas. Environmentalists were eager for some clarity on her position on the Keystone XL pipeline and fracking, but she instead said nothing on Keystone and repeated verbatim what she wrote in her book on fracking.

Steyer, who has not yet donated to the Ready for Hillary super PAC, added that he thought Clinton could benefit from a primary challenge. “Being forced to refine what you say and think is a good thing,” he explained. “Sometimes I think it can be debilitating and split the party, but in general I think Democrats are in good enough shape that we can handle a little internal discussion without falling apart. I think it’s very hard to go into a tough competitive [general] election cold.”

Steyer is the primary funder of the political group NextGenClimate, which had a large presence at the Iowa Steak Fry, where Clinton spoke Sunday. Ready for Hillary’s field director in Iowa also runs NextGenClimate’s work in the state.

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One of those who might challenge Clinton, Sanders, was also on hand. But the senator, who has steered clear of criticizing Clinton thus far, bristled before msnbc could even finish a question about the former secretary of state, who is in town for the Clinton Global Initiative.

“No, no, no, hold on. I resent that question, because it’s not a question,” he said. “I’m here because I believe that at the end of the day, the only way we make change – whether it’s on climate change, health care, income and wealth inequality – is by doing exactly what people are doing today, taking to the streets.”

Dennis Kucinich, the progressive former Ohio congressman and presidential candidate found Sanders and posed for a few pictures. Asked if Sanders should mount a bid, Kucinich told msnbc that it was a personal decision for the senator to make. “I served with Bernie Sanders in Congress, he and I were mayors together in the 70s, I’ve known him for over 40 years – Bernie’s a friend of mine and I certainly wish him well,” Kucinich said.