Now that former U.S. vice president Joe Biden has clinched the 2020 Democratic nomination for president, climate change is emerging as the crucial vote-determining issue on which he can win over skeptical progressive voters who backed Sen. Bernie Sanders in the party primary—as well as the 10% of 2016 Donald Trump supporters who can be persuaded to change their votes this year.

With key endorsements this week from Sanders and former president Barack Obama, multiple news analyses have concluded that Biden and the Democrats are in better shape to unify than they were four years ago, when Sanders didn’t endorse nominee Hillary Clinton until July. But while Sanders’ endorsement this time was personal and effusive, Biden’s winning coalition is still missing the “young and more liberal voters who overwhelmingly prefer Sanders’ call for political revolution to Biden’s promise of a return to ‘normalcy’,” The Guardian reports.

The Biden campaign is making overtures to the progressive wing of the party, engaging with key organizations and movement leaders in recent weeks. “Let me say, especially to the young voters who have been inspired by Senator Sanders: I hear you,” the candidate said, after sweeping three major state primaries last month. “I know what is at stake. And I know what we have to do.”

And now, “in interviews with progressive organizers, climate change has emerged as a key area where activists believe they can ‘push’ the former vice-president, in part because combating global warming draws broad support from every corner of the party,” The Guardian says. “Several polls this cycle have found that Democratic voters rank climate change among their top three issues, often second only to health care.”

“Joe Biden, as the moderate frontrunner, has to convince progressives to support him,” said Julian Brave NoiseCat, Green New Deal strategy director at Data for Progress. “He needs more enthusiasm behind his campaign, and it’s been a bruising campaign cycle.”

The Guardian says Data For Progress recently issued a recent six-page memo that listed five policy areas where supporters want Biden to come up with new platform planks or emphasize existing ones: stronger clean energy standards with clearer targets, investments in green infrastructure, dedicating 40% of climate money to disadvantaged communities, restricting Wall Street financing for fossil fuels, and a commitment to high-quality union jobs. “NoiseCat said the Biden campaign is engaging the authors at a high level,” the paper reports.

But at this moment, many climate voters are still looking for more “gutsiness and passion” in Biden’s position, 350.org North American Director Tamara Toles O’Laughlin told The Independent.

After a primary season that saw Sanders (I-VT) cast climate change as “a warlike situation” on which “we have to act dramatically”, Biden’s plan is “nowhere near enough”, she added. “As a climate-specific organization that’s focused on ending fossil fuels, his current positions are lacking ambition, to say the least.”

To win over those voters, billionaire climate campaigner and former primary candidate Tom Steyer said Biden would have to “convince[e] people that there’s a real emotional understanding and prioritization of things that are impacting people so deeply”. So far, he said, Biden’s climate plan doesn’t appear to be “fully formed. I’m sure it will come into much sharper focus over the coming months, hopefully the coming weeks, and that it will get a lot more specific.”

“We’re looking for a detailed plan for environmental justice,” O’Laughlin added. “Biden doesn’t seem to have the urgency, especially because we’re in a moment that will only be amplified by ongoing conditions in the climate.” The Independent has details on what 350.org is looking for in a candidate.

InsideClimate News says Biden “has offered a detailed roadmap for decarbonizing the economy that is historic by any number of yardsticks,” even though “it lacks the size, scope, and clarity of Sanders’ vision, embodied in the Green New Deal. Biden has called the Green New Deal ‘a crucial framework for meeting the climate challenges we face’, and his goal of net zero greenhouse gas emissions by 2050 is on par with Sanders’ and far beyond President Barack Obama’s pledge (an 80% reduction from 2005 levels.)”

Biden “has not embraced the bolder elements of Sanders’ plan—especially the melding of a national Medicare-for-all system into the climate package,” InsideClimate adds, and his campaign team includes an informal advisor linked to a major oil and gas fracking company. But his proposed US$1.7-trillion climate plan “includes 30 times the clean energy commitment in Hillary Clinton’s 2016 platform.”

Even so, “there remains a widespread feeling, even among his supporters, that he has yet to make a convincing case as a champion of climate action to young and progressive voters,” InsideClimate writes. “In the community of progressive climate activists, the overwhelming response to Sanders’ withdrawal was equal parts grief over the loss of their standard-bearer, and affirmation of their commitment to his ideals—underscoring the challenge for Biden.”

“We’re not going to sugar coat it: Our hearts are heavy,” Sunrise Movement spokesperson Aracely Jimenez, said in a statement. “The ball’s now in Joe Biden’s court. To avoid a repeat of 2016, he needs to show young people that he’s going to stand up for them by embracing policies like an ambitious Green New Deal that led young voters to flock to Bernie. If he doesn’t do this, our work turning out our generation to defeat Trump this fall becomes a lot harder.”

While Biden’s campaign embraces one essential outreach task, a recent study from Data for Progress and two Tufts University political scientists points to another opportunity to pick up climate votes. “The relatively small number of Trump voters—about 10%, according to the Cooperative Congressional Election Survey—who say they’re at least considering casting a vote for the Democratic nominee are a critical slice of the population to understand,” Vox writes. And “climate change could be the key for Democrats hoping to woo these voters.”

The potential changeover voters among Republicans tend to be younger, and “take a mix of liberal and conservative positions on various policy questions the CCES asks about,” writes reporter Matthew Yglesias. And among those various issues, “climate and immigration stand head and shoulders above the rest”. Between the two of them, climate is the one where “wavering Trump supporters are clearly aligned with Democrats,” with large majorities supporting renewable energy mandates and federal regulations on greenhouse gas emissions, and opposing Trump’s move to appeal the Obama Clean Power Plan and withdraw from the 2015 Paris agreement.

“Most Democratic operatives seem skeptical of the political merits of the climate issue,” Yglesias writes, “but the evidence suggests that it—rather than the aberrant personality or corruption topics that often dominate negative media coverage of Trump—is one of the biggest drivers of doubts about the president among people who supported him in the past. Finding ways to inject the issue into both paid and earned media to remind waverers of the large climate stakes in the election could be a key way to turn those doubts into votes.”