After airing their differences in the primary fight between Clinton and Bernie Sanders, green groups seek common ground because the alternative is Donald Trump.

When the final balloons dropped on the political conventions, the environmental movement found itself surrounded by more than just some trash to sweep up. The surprising strength of Bernie Sanders’ candidacy had clearly pulled the Democratic nominee, Hillary Clinton, and the party platform to take a far stronger stance on climate change and environmental issues, but it also left many Sanders supporters disillusioned and the party’s normally reliable coalition of green group support wrestling with a big question: What now?

The general election ahead is, by some predictions, going to be decided by a slim margin—most polls have Clinton with a slight lead over Donald Trump following the Democratic National Convention. And that could give the climate issue, which has failed to register heavily in any national election so far, the potential to make a difference in key states.

That puts the spotlight on the green groups that spent the primary season at odds and now are trying to figure out a way to work together. Because even if some of them haven’t come around to an endorsement of Clinton, they grasp the ramifications of electing a full-throated climate denier to the White House.

“The choice couldn’t be more stark this year and the stakes couldn’t be higher,” said Tiernan Sittenfeld, senior vice president of government affairs for the League of Conservation Voters (LCV). “It’s just inconceivable to me that anyone [who cares about climate] would decide they want to support Donald Trump.”

The effort to rally environmental voters behind Clinton began as early as last fall when she won the endorsement of the political arm of LCV, a group with a broad base of 1.1 million members. But while Sanders made the primary fight far more competitive than many expected, the “Bernie or bust” camp were still struggling with the idea of supporting Clinton up to the convention.

They did manage to organize a march in Philadelphia that included several thousand people. It also helped that one of the “keep it in the ground” movement groups, 350.org, has begun to soften its stance on Clinton. The group’s co-founder Jamie Henn took to Twitter from the convention to suggest a viable path for progressives to get behind the party nominee.

Seeking Common Ground

A mostly informal coalition of greens has begun to coalesce around Clinton, with mainstream groups like LCV, Sierra Club, the NRDC Action Fund and NextGen Climate playing a leading role. But they’re sharing strategy with both the most assertive groups, like Climate Hawks Vote, and the organizations that are more willing to partner with industry, like the Environmental Defense Fund (EDF). “We’re working with everyone we can,” said Sierra Club executive director Michael Brune. “We’ll work with Climate Hawks Vote. In many ways, we’ll work with EDF when our interests overlap. We’ll work with anybody who is supporting a true and genuine climate champion.”

Hillary Clinton, Brune and the others argue, is that kind of candidate. The green groups are employing age-old politicking techniques. (LCV volunteers knocked on doors for Clinton in key primary states like Iowa and New Hampshire that now are general election battlegrounds.) And they’re experimenting with new ones. (NextGen Climate is testing the power of Pokemon Go to lure voters to discussions on the importance of voting.) Their campaigners are trying to coordinate endorsements down ballot, like a joint announcement of support in March for Tammy Duckworth in the Illinois Senate race, and new talking points (the Sierra Club, for example, letting colleagues know about its new research showing that if elected, Trump would be the only world leader who rejects climate science).

“We share information about the choices we’re making in prioritizing Congressional races, messages we’re advancing, reports that we put together,” said Brune. “We don’t always agree—no coalition does—but communication is pretty tight.”

Some of the political coordination is formal, like the LeadingGreen alliance that LCV and the NRDC Action Fund established before the 2014 midterm elections—refocused and amped up for 2016. LeadingGreen is raising and bundling campaign contributions—just as industry lobbyists long have done. Its target is to surpass the $6 million it raised during the midterms, and the groups already have been able to put together $4.5 million for Clinton and other pro-environment candidates.

“We’re picking up the pace, because environmentalists and environmental donors all across the country feel the urgency of this election,” said Craig Auster, LCV’s director of advocacy partnerships.

If they worked together, there’s no question that the nation’s environmental organizations could command considerable clout. The 10 largest green groups have 15 million members and a combined annual budget of $525 million, according to an ICN analysis from 2014.

For comparison, the National Rifle Association claims only 4.5 million members (a number that may be inflated), and an annual budget of $345 million.

Bitter Memories of Elections Past

But only a few months ago, it seemed unlikely environmentalists would be able to forge a united front. LCV faced a barrage of criticism from the movement’s Sanders supporters when it bestowed on Clinton the earliest endorsement it ever had made for a presidential candidate in its 47-year history, and its first made before any votes were cast. But the decision was in part born out of LeadingGreen’s bitter experience in 2014.

Despite green group money-raising and campaigning that year, the results that November were devastating for environmentalists, with the GOP seizing control of the U.S. Senate for the first time in seven years. Among the losers were four Democratic Senate candidates that LeadingGreen backed: former U.S. Rep. Bruce Braley of Iowa, nonprofit executive Michelle Nunn of Georgia, and incumbents Mark Udall of Colorado and Kay Hagan of North Carolina.

“Obviously, it was a bad cycle for pro-environmental candidates, but we learned a lot of good lessons,” said Auster. One important takeaway, he said, was that they needed to start campaigning earlier. “All the great things that President Obama has done for climate—the Clean Power Plan, [the decision to block the] Keystone XL pipeline, the Paris climate agreement—are on the line,” he said. “And this election is going to be hard and tight. We felt it was so important to get in early and unite behind the candidate who could win and would build on the progress President Obama made.”

After endorsing in the fall, LCV put out a call for volunteers to work directly with Hillary for America during the primaries in a program nicknamed GreenRoots. (Some of LCV’s political activities are coordinated directly with the Clinton campaign, which the group is allowed to do by law as long as its Super PAC is not involved.)

More than 270 GreenRoots members worked phones and pounded the pavement, contacting some 45,000 voters in five states, all of which polls suggest will be battlegrounds in the general election: Iowa, New Hampshire, Colorado, Nevada and Pennsylvania. Clinton’s narrow win in the Feb. 19 Nevada caucuses was widely seen as a turning point that dampened Bernie Sanders’ momentum, and, said Auster, “We feel we made a contribution to her narrative-changing victory.”

There’s a practical advantage if green movement players can demonstrate their value in helping Clinton get elected. It earns them a place at the policymaking table, and even a shot at having their people tapped for key administration posts.

Support Mixed with Caution

But for the green groups that favor outside-the-Washington-beltway activism, don’t expect outright endorsements for Clinton.

350.org works closely in networks with indigenous groups, U.S. Gulf Coast communities, and others that are fighting fossil fuel development in their backyards. These are places where Bernie Sanders’ call for a fracking ban often resonated more deeply than Clinton’s more deliberate approach of regulating fossil fuel development while encouraging the transition to clean energy. Robert Gardner, 350.org’s national political coordinator, said his organization’s devotion is to those communities and the movement, not to any one candidate

But in a tone markedly different than some of the rhetoric from the left earlier in the campaign season, Gardner declined to criticize mainstream green groups for taking a different approach. “It’s sort of like different opinions in a family,” said Gardner. “Other environmental organizations have different calculuses they use to determine whether or not to endorse, and when. But we keep the family discussions civil.”

Gardner said his organization has “sincere questions” about Clinton’s stance on oil and natural gas development, but he offers what can only be described as a non-endorsement endorsement. “We recognize that the potential damage from a Trump presidency is almost existential,” he said. “Organizationally, we see us rolling up our sleeves and making sure that every single individual that is listening to 350 or is within earshot knows we’re expecting them to show up in the November.”

A meeting of greens from across the political spectrum took place on Tuesday during the Democratic convention, a “Winning on Climate Together” reception at the historic Warwick Hotel in Philadelphia.

The gathering was co-hosted by LCV, the Sierra Club, NRDC Action Fund, and NextGen Climate, the political action group founded and financed by California billionaire Tom Steyer. Speakers included two Democratic candidates in the forefront of the drive to retake the U.S. Senate: Katie McGinty of Pennsylvania, whose prominence as a state and federal environmental official dates to her time as an aide to Al Gore; and Ted Strickland of Ohio, the former governor who spent a year as president of the fiercely pro-climate action Center for American Progress Action Fund. While the pair represented the political mainstream of the climate action drive, the podium also featured leaders of frontline communities and indigenous groups who represent the broadening of the movement. Representatives of Friends of the Earth, a group that had endorsed Sanders, were in attendance, and they tweeted a picture of the Hip Hop Caucus president, Rev. Lennox Yearwood, urging the gathering to see climate change as “the human rights movement of the 21st Century.”

But winning over Sanders supporters remains one of the biggest challenges for climate activists backing Clinton, according to the green group organizers. NextGen Climate plans to spend $25 million to reach the voters who were Sanders’ base: 18-to-34-year-old voters. NextGen will fan out to more than 200 college campuses in seven battleground states in what it said is the largest youth voter turnout effort ever undertaken in U.S. history. The group also is trying new ideas for reaching the millennial generation, like holding Pokemon Go meet-ups where young people can capture rare Pokemon and hear about the importance of committing to vote for clean energy candidates.

During the DNC, NextGen released its own polling indicating significant numbers of millennial-generation voters in swing states may be “Sanders holdouts”— planning either to sit out the election or vote for a third-party challenger, such as Green Party candidate Jill Stein. Although the online poll may not have captured a representative sample, the possibility of such holdouts is especially worrisome to those in the movement who remember how the Green Party candidacy of consumer rights icon Ralph Nader in 2000 fatally wounded Democratic Vice President Al Gore’s presidential bid. Although Nader only garnered 2.7 percent of the nationwide vote, the 97,888 votes he took in Florida easily could have given Gore the race, while in New Hampshire, Nader received 22,198 votes, more than three times George W. Bush’s 7,221-vote margin of victory.

‘A Frightening Contrast’

RL Miller, president and cofounder of the fiercely progressive Climate Hawks Vote, who said more than 90 percent of her members supported Sanders, said the group won’t be endorsing Clinton, but will be talking to members about the election’s high stakes. “I’m going to tell my list, primarily the Bernie Sanders people, that voting in this election in a way that causes Donald Trump to be elected means the Paris agreement goes way of Jimmy Carter’s solar panels,” Miller said. “And unlike those panels, which were symbolic, the Paris agreement is actually real.”

Miller said the desire to lodge a protest vote is understandable. But “given the choice between a sane, competent person who will work on the Paris agreement and will work on fracking regulations, and someone who wants to undo all of these with the stroke of a pen, and thinks he will have sufficient executive power to do so?” Miller asked. “It’s a frightening contrast.”

If both mainstream environmental groups and progressives joined in rallying around Clinton’s candidacy, it would echo the playbook of perhaps the movement’s most successful recent campaign: the defeat of the Keystone XL pipeline project. Keystone XL is “a fabulous example” of what the environmental movement can do when working together, said Kevin Curtis, executive director of NRDC Action Fund, the political affiliate of the Natural Resources Defense Council. He also thinks the coalition’s success in shifting the Obama administration’s stand holds lessons for those who are disappointed that Clinton’s climate policy doesn’t go far or fast enough.

“Put me in the category of ‘been in Washington too long,’” said Curtis. “I didn’t think we’d win on Keystone. I didn’t think it was a fight worth having. Well, I was wrong, and I’m thrilled. So I think part of the message to millennials and Sanders supporters is a great big ‘thank you’ for having brought such energy and passion to this topic and having moved the whole debate. So now, let’s for the next few months focus on getting a much friendlier team elected, and then we’ll continue to put pressure on them.”

Brune is optimistic that the vigor that Sanders voters showed in pushing the Democratic party on climate issues during the primary election will propel them to vote for Clinton in the fall. “We have loved the fact that there is a growing base of climate voters who are organized, sophisticated, and who were persistent in pushing for the strongest possible platform,” he said. “Now, with the choice between Clinton and Trump, we’ve never had in the history of presidential politics a gap this large on climate and environmental issues

“That’s a choice that won’t be difficult,” said Brune.