On paper, Joe Biden has the most aggressive plan to address climate change ever proposed by a Democratic nominee.

But liberal climate activists, many of them young supporters of rival Vermont Sen. Bernie Sanders dejected by his loss, see Biden's pitch as too generic and vague, and his delivery uninspiring.

These activists aim to exert pressure on Biden during his campaign for the general election, pushing him to add detail to his headline target of the United States using 100% carbon-free energy by 2050 and to set bolder near-term targets to make progress on weaning off fossil fuels.

“The Biden plan is the most ambitious climate plan any nominee has ever put forward, including Bernie in 2016,” said Bracken Hendricks, a former senior climate policy adviser in the presidential campaign of Jay Inslee, the governor of Washington. “With that said, the climate crisis is worse and moving more rapidly, so the urgency of the moment is changing the debate.”

While Biden assembled a broad primary coalition of older suburban and blue-collar voters, along with African Americans, climate activists say he’ll need turnout and enthusiasm from young voters to beat President Trump.

“There is a generational divide in this country that's very real,” said Brandon Hurlbut, a former campaign adviser for Obama's 2008 campaign and the chief of staff at the Energy Department. Hurlbut volunteered in 2020 for the presidential campaign of Massachusetts Sen. Elizabeth Warren but is backing Biden now. “The Democratic Party must be responsive to young voters' desire for aggressive action on climate now,” he said.

Biden is aware of his enthusiasm problem with young climate voters who supported Sanders. He appealed to their concerns after notching primary wins in Florida, Illinois, and Arizona on March 17, telling them, "Especially to the young voters who have been inspired by Senator Sanders: I hear you. I know what is at stake. And I know what we have to do.”

After Sanders endorsed Biden last week, the two announced they are creating a task force on climate change.

But the youth-driven Sunrise Movement, the influential group that helped bring attention to the Green New Deal, has not endorsed Biden.

“Hopefully [Biden] will continue working prominently with [Sanders] throughout the campaign, and these tasks forces will produce real progressive policies & personnel commitments, not just empty gestures,” tweeted Evan Weber, the Sunrise political director.

Progressive star New York Rep. Alexandria Ocasio-Cortez, who is aligned with Sunrise, has also not endorsed Biden and cited climate change as an area she would like to see him improve.

Nonetheless, Biden’s staff is talking with liberal groups and activists, Hurlbut and others in touch with his campaign say.

There's an expectation Biden will be releasing additional climate plans with more detail in the coming months.

Hendricks and others say Biden has an opportunity to lay out a more proactive vision for responding to climate change as the nation recovers from the coronavirus pandemic. Eight youth-backed liberal groups, including Sunrise, NextGen America, and Justice Democrats, wrote a letter to Biden this month imploring him to commit to "mobilizing $10 trillion in green stimulus and infrastructure investments over 10 years that will create tens of millions of good jobs of the future." Hendricks says that money should be focused on water systems, assistance for low-income utility rate-payers, power grid modernization, expanded public transit, and more.

“In this moment of economic recovery, there is a lot more for Biden to say,” Hendricks said. “There is a hunger to see how the economic vision builds on his climate plan.”

Biden seems to be hearing that message.

He called for a “green deal” stimulus last month. As vice president, Biden helped implement President Barack Obama’s Recovery Act of 2009, dedicating $90 billion to clean energy programs that lowered the cost of wind and solar.

But if Biden wants to catch Sanders, he’d have to spend a lot more money: Sanders’s climate plan envisioned spending $16 trillion, the most of any candidate, while Biden has proposed $1.7 trillion over a decade.

“People would like to see a bigger investment from Biden,” said Leah Stokes, an assistant professor of environmental politics at the University of California at Santa Barbara, whose research has informed the campaigns of various candidates. "The recent coronavirus stimulus packages that have gone through show that spending a trillion dollars over 10 years is not that much money."

It's not that "the Biden plan has to become the Bernie plan," Hendricks said.

Hendricks and other liberal activists aren't necessarily demanding Biden adopt some of Sanders’s more controversial proposals that could damage him in swing states, such as immediately banning fracking, a drilling technique for extracting oil and gas deep underground.

"Honestly, even talking about fracking vs. not fracking is sort of missing the point. Who cares how you get the oil and gas," said Rep. Jared Huffman, a California Democrat and a member of the House Select Committee on the Climate Crisis. "The point is you need to make a dramatic break from fossil fuel altogether. That's where forward-leaning climate advocates would like to see more from Biden."

Sanders’s plan, in contrast to Biden’s, sought to have the nation’s power and transportation run entirely on renewables by 2030, while ending the use of fossil fuels in the economy by 2050. Biden leaves the door open for fossil fuel use, even out to 2050, if the emissions from a gas plant, for example, can be negated with carbon capture technology.

“Biden's plan talks only of a 100% clean energy economy by 2050 when he will be 107 years old and most likely dead,” said R.L. Miller, founder of the liberal voter mobilization group Climate Hawks Vote. “When his plan includes a road map of how to get to that goal, with 2030 as a way station along the route, I'll take it seriously.”

Activists are urging Biden to detail how he'd make progress toward reducing fossil fuel use over the coming decade. They want him to specify how he would reach his goal of net-zero emissions by 2050, and to set near-term targets for decarbonizing the economy’s various sectors, including electricity, transportation, and buildings.

“If you punt everything to 2050 as your only goal, it's punting the issue to the next generation,” said Brett Hartl, chief political strategist of the Center for Biological Diversity Action Fund.

Biden faces risks if he wades deeper into climate policy and illuminates a dire future for fossil fuels. His blue-collar background and relatively centrist views have managed to not offend unions whose workers would suffer from eliminating fossil fuel jobs.

As liberals look to move Biden left, Trump is attacking from the right, eager to portray Democrats as out-of-touch when the country is battling the coronavirus pandemic.

“I am not worried at all that by leaning in on climate, Biden would risk the election,” said Huffman, who recently endorsed Biden. “Leaning in helps to highlight how badly out of step Trump is on this issue.”

There’s also a question of whether Biden needs to do anything more to demonstrate his climate bonafides during an election that will likely turn on how voters view Trump and his handling of coronavirus.

Biden was the preferred candidate of voters who said climate change is the most important issue to them on Super Tuesday, according to exit polls. But that could mean little since these climate-minded voters likely saw Biden generally as the best candidate to beat Trump.

“This election will really be a referendum on Trump,” Hartl said.

Embracing more progressive positions to appease young voters could cost more than it helps, given their poor turnout record.

“Young voters need to understand that elections matter,” Hurlbut said.

Hurlbut predicted climate-minded young voters would come around to Biden if the former vice president initiates an inclusive conversation and hears them out.

“Everyone knows the stakes of this election,” he said. “This is not either-or. It’s a game of addition that will require Biden to unite all of these coalitions. We need all of them to win.”