(CNN) Rep. Alexandria Ocasio-Cortez on Monday made the case for radical action to combat climate change -- and took a shot at former Vice President Joe Biden, questioning his commitment to the issue.

"I will be damned," Ocasio-Cortez said at a rally, "if the same politicians who refused to act (in past decades) are going to try to come back today and say we need a middle of the road approach to save our lives."

The comments from Ocasio-Cortez, who spoke at the event after Democratic presidential candidate Sen. Bernie Sanders, channeled the left's doubts that Biden is committed -- or inclined -- to push for the kind of comprehensive economic and social overhaul called for by the Green New Deal.

The comment, which appeared to be aimed directly at Biden, came days after Reuters reported the former vice president planned to roll out a plan that one member of the team advising him on climate policy described as answering a need to "find middle ground" on the issue.

Six months after their headline-grabbing sit-in at House Speaker Nancy Pelosi's Capitol Hill office, leaders of the Sunrise Movement, a millennial group spearheading the campaign for the Green New Deal , hosted Ocasio-Cortez, Sanders and other activists on Monday night.

The show of organizing force, which packed the Cramton Auditorium at Howard University, was designed in part to send a message to wobbly or skeptical Democrats -- like Biden -- who have yet to embrace the framework of the radical proposal. The Green New Deal calls for a "10-year mobilization to achieve greenhouse gas neutrality" and is a comprehensive vision that would fundamentally overhaul the US economy. It also includes a jobs guarantee and other expensive provisions designed to protect American workers during the transition.

Biden said progressives criticizing him over his commitment to combating climate change "should calm down a little bit."

"Take a look at the record before. There's nothing been the middle of the road about my effort in dealing with the environment. Nothing," he said.

His comments came the day after Biden cast climate change as an "existential threat" during a campaign event, and pledged to roll out his own climate policy proposal in the coming weeks.

The climate change activists in the auditorium on Monday night, though, had seized on the Reuters report -- and held it up as evidence against any moderate criticism of the Green New Deal.

"There is no 'middle ground' when it comes to climate policy," Sanders tweeted after the Reuters report came out.

"If we don't commit to fully transforming our energy system away from fossil fuels," he continued, "we will doom future generations. Fighting climate change must be our priority, whether fossil fuel billionaires like it or not."

Stephen O'Hanlon, a spokesman for the Sunrise Movement, told CNN on Monday morning that a "middle ground approach that means more fossil fuels and more oil and gas development is a death sentence for our generation and millions of people around the world."

But he wasn't ready to write off Biden -- at least not yet.

"We're nervous about where he stands," O'Hanlon said, "but there's time for him to do the right thing."

The Sunrise Movement's public ascent began late last year, shortly after the Intergovernmental Panel on Climate Change released a jarring report that declared "there is no documented historic precedent" for the rate and scale of reform needed to blunt the current pace -- and potential deadly impacts -- of global warming.

In the months since, climate change has vaulted to the forefront of the debate within the Democratic Party and among its 2020 presidential primary contestants. A series of recent natural disasters -- from the California wildfires to flooding in the Midwest -- and new reports, including one from the United Nations this month that found one million species at risk of extinction because of human activity, have combined to grow the sense of crisis into the mainstream.

The issue topped a CNN poll of top issues for Democrats and Democratic-leaning independents last month, with 82% saying it was "very important" that the candidates support "taking aggressive action to slow the effects of climate change." Only 43% said the same when asked about "impeaching Donald Trump."

Sunrise-aligned activists are planning mass protests urging the Democratic candidates to back it during the July primary debates in Michigan. But the plan has met resistance from the party's moderate quarters.

In an interview before his speech, which called for an end to "all subsidies and tax breaks for the oil and gas companies," Sanders brushed back the Green New Deal's Democratic skeptics.

"When you're dealing with the future of the planet and making sure that our kids and our grandchildren have a healthy and habitable world in which to live, I don't know how you go too far," Sanders said. "This is an existential threat, not just to the United States, but to the entire planet."

Sanders called President Donald Trump's position on climate change "sad" and said the administration, which has sought to install skeptics and deniers in departments like the Environmental Protection Agency, is making "a very bad situation even worse."

The rally also marked the second occasion in less than a week that Ocasio-Cortez and Sanders, though they never shared the stage here, came together to push for a common cause. Ocasio-Cortez on Thursday named Sanders and Massachusetts Sen. Elizabeth Warren as two candidates who were in the running for her endorsement, which she said was still likely some ways off.

Asked on Monday if he planned to ask for her support, Sanders balked.

"Right now we are here to talk about the environment," he said.