Joe Biden capped off his stunning political comeback Wednesday, overcoming two failed presidential bids and months of organizational, fundraising and messaging struggles to become the presumptive Democratic nominee. But the challenge of uniting the party around him is just beginning.

The decision by Sen. Bernie Sanders, the progressive champion and Biden’s chief rival, to drop out of the Democratic race left Biden with the mighty task of energizing the many younger, more liberal voters who powered Sanders’ campaign and have long felt tepid at best about the former vice president — and at worst, have harbored deep antipathy toward him.

Now Biden and his allies are hoping to quickly bring the party’s divided factions together after spending much of the past two weeks quietly negotiating with. Sanders’ team to find common ground on the senator’s policy priorities as he mulled an exit.

Biden and Sanders spoke directly midday on Wednesday, according to people familiar with the conversation. The Biden campaign is expected, starting on Thursday, to highlight a series of policy positions that show how he has moved closer to Sanders on health care, climate and other issues, and the two camps are continuing to negotiate other details, like the establishment of policy working groups, according to people with direct knowledge of the plans. During a virtual fundraiser Wednesday, Biden suggested he would also expand his climate change proposals to accommodate the views of progressive groups.

There is also the possibility of a full-fledged Sanders endorsement of Biden, and potentially a joint online appearance, although the details remained in flux on Wednesday, these people said. Biden’s campaign also had a prescheduled call for top contributors shortly after the news broke that Sanders had exited the race. His advisers were pressed at one point about the campaign’s plans to reach out to young voters, according to a person on the call.

Biden wrapped up the primary faster than many had expected, washing away concerns that Democrats would be divided deep into the summer. Now he has a chance try to unify the party while President Donald Trump is struggling to contain the coronavirus pandemic and its political fallout.

“Thank goodness we can finally get to work,” Biden’s wife, Dr. Jill Biden, told donors in a virtual coffee event hours after Sanders quit the race.

Yet the challenges Biden will face in exciting Sanders’ devoted base — at a moment when Trump has an iron grip on the Republican Party — were on vivid display within hours of the senator’s withdrawal.

Several youth-focused organizations, some of which had previously endorsed Sanders, sent an open letter to Biden “expressing concern over his inability to earn the trust of the vast majority of voters under 45 years old.”

The letter, signed by groups including Justice Democrats, the Sunrise Movement and March for Our Lives Action Fund, sought to pressure Biden’s team in two key areas: policy and personnel. On the policy front, the letter urged Biden to adopt expansive proposals like the climate-focused Green New Deal, the elimination of the Senate filibuster, and free college tuition. It also asked Biden to include elected officials who backed Sanders or Sen. Elizabeth Warren on important campaign policy teams and in strategy decisions.

The organizations “will spend more than $100 million communicating with more than 10 million young members, supporters, and potential voters this election cycle,” the letter read, “but we need help ensuring our efforts will be backed-up by a campaign that speaks to our generation.”

It’s a constituency Biden’s campaign all but abandoned targeting during the Democratic primary, at least online. On Facebook, he spent less than 1% of his ad budget targeting voters under 25, according to data compiled by Bully Pulpit Interactive, a Democratic firm, by far the smallest fraction in the field.

But Biden and his team say they recognize the need to improve his standing with the younger and more progressive voters who flocked to Sanders in both the 2016 and 2020 presidential campaigns — and quiet efforts to engage them, and Sanders himself, have been underway over the past month.

Conversations over health care, climate change and other policy matters culminated in a week of intense conference-call negotiations among top advisers from each campaign. Sanders’ top political adviser, Jeff Weaver, and his campaign manager, Faiz Shakir, negotiated on behalf of the Vermont senator, and two longtime Biden advisers, Anita Dunn and Ron Klain, represented the former vice president, according to several people close to the situation.

A number of Biden’s advisers and surrogates have also reached out to a constellation of liberal organizations and leaders, calling for unity against Trump and making the case that Biden has a progressive policy vision, while also seeking input and hoping to signal a willingness to listen.

The campaign is “ensuring folks have the facts, engaging in good-faith dialogue, so we can come to some actionable items,” said Symone Sanders, a senior adviser to Biden who has helped lead the progressive outreach. “I think we will get there with a number of these groups and I think we’ll get there sooner than later.”

There is also an internal working group focused on younger voters, and Biden’s advisers said Wednesday that constituency was a top priority. And Rep. Cedric Richmond of Louisiana, a national campaign co-chairman for Biden, has maintained a dialogue with his counterpart on the Sanders campaign, Rep. Ro Khanna of California.

Some of Biden’s allies have also started to think about issues on which he could move further to the left. The coronavirus outbreak has sharpened attention on economic inequality and health care disparities, allies say, and while Biden has been clear that he does not support Sanders’ sweeping single-payer health care proposal, “Medicare for All,” supporters say he could make substantive overtures to progressives in other ways.

“The vice president has a very good policy right now when it comes to health care, in terms of a very strong public option,” said Rep. Ruben Gallego, D-Ariz. and a progressive Biden supporter. “Talking to Bernie people, they would like to see a stronger universal health care package. It doesn’t necessarily have to be Medicare for All, but something that brings more people more coverage. And I think the vice president, if he moves that direction, will find a lot more support among progressives.”

Biden’s campaign did not comment on whether he was weighing changes to his health care position. But while he is not expected to embrace Medicare for All, he has adopted a number of other proposals championed by Sanders as well as by Warren, another progressive former presidential candidate.

With Sanders out of the race, one of Biden’s urgent tasks is to close his financial gap with Trump’s campaign and the Republican National Committee, which entered March with a combined $225 million cash on hand. Biden had $12 million and the Democratic National Committee had $14 million, with $6 million in unpaid bills.

Biden’s campaign and the DNC are expected to have imminent conversations about setting up a joint committee, said Xochitl Hinojosa, the DNC’s communications director. Inside the Biden finance operation, those conversations have already begun.

Until an agreement is reached, Biden is limited to soliciting $2,800 checks each for the primary and the general election; a shared DNC account could allow him to request contributions for hundreds of thousands of dollars.

But beyond such megachecks, the bigger financial concern for Biden is his ability to attract small donations online, a particularly acute need now that the campaign is operating almost entirely virtually.

Through February, Biden had spent less than half as much as Sanders, a testament both to his strengths as a candidate, given that he won the fight, and to his weakness in attracting small donations.

David Wilhelm, a former chairman of the Democratic National Committee who worked on Biden’s 1988 presidential campaign and is now a donor, said the Sanders exit represented a sigh of relief.

“My greatest fear going into this election cycle is that we would have a bruising battle for the nomination and we would spend so much of our energy and so much of our resources on an internal party battle that by the time the convention rolled around we would be out of money and out of energy,” Wilhelm said.

Biden aimed to avoid leaning on Sanders to withdraw from the race, frequently saying that he understood the difficult and personal nature of the decision. And in sharp contrast to the rancor of Sanders’ withdrawal in 2016 after the bruising primary against Hillary Clinton, the direct talks between the two camps were eased by the cordial, at times friendly, relationship between the two principals.

Sanders has told people close to him that he appreciated the fact that Biden did not overtly pressure him to quit after Super Tuesday, and praised Biden for “not hitting me below the belt” when he trailed Sanders in the polls early this year.

Yet other progressives made clear on Wednesday that Biden had considerable work to do.

“If Joe Biden is going to be the nominee on the Democratic side, we are going to champion Joe Biden and put all muscle behind” those efforts,” said Varshini Prakash, the executive director of the Sunrise Movement. But, she added, “You haven’t earned our vote yet.”