While debating in Miami—the U.S. city arguably most threatened by rising sea levels caused by the warming of the planet—Wednesday night, presidential candidate Bernie Sanders challenged Democratic rival Hillary Clinton to strengthen her positions on climate change by joining him in a call for a tax on carbon and a total ban on fracking nationwide.

During the latter part of the debate, when asked by a CNN moderator to discuss his position on climate, Sanders said, "Let’s be clear. You’re looking at the senator who introduced the most comprehensive climate change legislation in the history of the United States Senate."

He then added, "Now, I hope that Secretary Clinton would join me if we are serious about climate change, about imposing a tax on carbon on the fossil fuel industry and making massive investments in energy efficiency and sustainable energy. And by the way, while we are on the subject of energy, I hope you’ll join me in ending fracking in the United States of America."

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The moderators did not give Clinton a chance to directly respond to Sanders' challenge on those points. Still, members of the climate movement expressed concern about the positions Clinton continues to hold when it comes to climate change—including specific answers she gave Wednesday night.

When asked about the issue, Clinton defended President Obama's record on climate—even though scientists and climate experts have slammed his "all-of-the-above" energy approach—and said she would do everything she could to continue those policies while also pushing for Republicans to finally stop denying the existence of the problem and join the fight. "We need to implement all of the president’s executive actions and quickly move to make a bridge from coal to natural gas to clean energy," Clinton said. "That is the way we will keep the lights on while we are transitioning to a clean energy future. And when I talk about resilience, I think that is an area we can get Republican support on."

In response, however, climate advocacy group 350 Action spokesperson Jamie Henn voiced skepticism about Clinton's stance.

"Hillary Clinton still needs to convince climate voters that she’s willing to stand up to the fossil fuel industry," said Henn following the debate. "I cringed when I heard her mention natural gas as a bridge fuel. Scientists are now clear that natural gas is a bridge to nowhere and that we must move directly and swiftly to 100% renewable energy. Clinton is coming out stronger on climate, but her ties to the fossil fuel industry are still a major cause for concern."

Despite those reservations, 350 Action said Wednesday's debate was a testament to how far grassroots activists have moved both candidates on climate change. The group believes that as the primary contest with Sanders has gone on, Clinton has endorsed increasingly progressive stances on climate change, such as coming out against the Keystone XL pipeline and limiting fossil fuel development on public lands. Meanwhile, Sanders has adopted even stronger rhetoric, incorporating the language of the "keep it in the ground" movement which, led by students and a vast coalition of climate activists in the U.S and around the world, is calling on millions of people to rise up and take on the fossil fuel industry.

"We’ve seen a huge shift over the last year," said Henn. "Democrats have gone from an ‘all of the above’ strategy on climate to all out opposition to the fossil fuel industry. That’s a sign of the growing strength of our movement and the growing realization that the true test of climate leadership is keeping fossil fuels in the ground."