Until Tuesday, Barack Obama was the missing guest in the Joe Biden campaign, the empty chair that was all the more conspicuous because you couldn’t help wondering when it was going to be filled. Biden built his entire campaign on his having been Obama’s vice president, a tenure that earned him enough support from Democratic Party loyalists for him to become the party’s presumptive nominee. While Biden was never shy about mentioning his friendship with “Barack,” Obama himself, wary of taking sides, withheld his support until after Biden’s last major competitor, Bernie Sanders, dropped out of the race and himself endorsed Biden.

Obama reportedly worried that if he championed Biden, the primaries would be seen as a reprise of 2016, when many Sanders supporters felt the Democratic establishment had cleared the field for Hillary Clinton and given her an unfair advantage.

This concern for party unity is understandable. The Democratic Party has splintered since Obama’s two terms as president. All the major Democratic candidates appeal to fragments of the successful Obama coalition but haven’t been able to pull together the entire package. Clinton succeeded in expanding parts of the Obama coalition by bringing in more suburbanites, often disgruntled Republicans. But this came at the cost of doing worse with other elements of the Obama coalition, especially young people. Biden has won the support of African Americans, suburban moderates, and working-class whites but, like Clinton, has had trouble with young voters. Indeed, Biden rarely won voters under 45 in most contests. Sanders’s campaign failed because it couldn’t attract sufficient African American support, but he did very well with Latinx and other people of color, in addition to young people.

The inability of any candidate in 2016 or ’20 to replicate the Obama coalition makes the former president more important than ever. He is perhaps the only politician who is warmly regarded by almost all the major factions in the party, except perhaps the most passionate Sanders supporters.

It’s a telling sign of how divided Democrats remain that a significant chunk of Obama’s endorsement was taken up by an attempt to win over Sanders supporters with a combination of magnanimous praise for the Vermont senator and also promises that a Biden administration would be progressive.

Each of our candidates were talented and decent with a track record of accomplishment, smart ideas, and serious visions for the future, and that is certainly true of the candidate who made it farther than any other, Bernie Sanders. Bernie is an American original, a man who has devoted his life to giving voice to working people’s hopes, dreams, and frustrations. He and I haven’t always agreed on everything, but we have always shared a conviction that we have to make America a fairer more just, more equitable society.

We both know that nothing is more powerful than millions of voices calling for change and the ideas he has championed, the energy and enthusiasm he inspired especially in young people will be critical in moving America in a direction of progress and hope because for the second time in 12 years we will have the incredible task of rebuilding our economy and to meet the moment the Democratic Party will have to be bold.

Remarkably, Obama acknowledged that the Democratic Party would have to accept policies further to the left than what he implemented as president. “You know I could not be prouder of the incredible progress that we made together during my presidency, but if I were running today, I wouldn’t run the same race or have the same platform as I did in 2008,” Obama said. “The world is different; there’s too much unfinished business for us to just look backwards. We have to look to the future. Bernie understands that, and Joe understands that.”

Obama’s endorsement was a remarkable performance, and a reminder that he’s still an outsize talent in American politics. It was eloquent, large-hearted, and farsighted.

But was it persuasive?

Let’s stipulate that Obama’s key argument, that Democrats need to unite to defeat Trump, is accurate. There still remains the fact that Biden is an imperfect leader for a popular front. It’s true that Biden’s lack of ideological rigidity allows him to shift to the left, just as he had earlier shifted to the right in the 1980s and ’90s.

But there’s a difference between accepting policies on a rhetorical level and convincingly adopting them as positions you would fight for. The Joe Biden we saw on the campaign trail lacked the agility of Obama and the ability to show empathy for those trying to push him further to the left. He was visibly annoyed at activists who were trying to prod him to take bolder stances on climate change and immigrant rights.

Obama tried to address the issues of Biden’s limitations by suggesting that his administration would be coalition with strong advisers. But who would those advisers be? According to Obama, Biden “will surround himself with good people,” such as “experts, scientists, military officials.” This is a technocratic vision of politics, one that has a real appeal amid Donald Trump’s incompetence. But it is also a very narrow view of staffing. There’s no assurance from either Biden or Obama that the next Democratic administration would also have powerful posts for the progressive activists who rallied behind Bernie Sanders. But unless there are progressives inside the administration, all the policy promises are unlikely to be implemented.

While Obama remains enormously popular among Democrats as a whole, there is a small subset of hard-core Sanders supporters who are likely to remain immune to his blandishments. These Sanders supporters believe that despite Obama’s ostensible neutrality in the Biden-Sanders fight, the former president was working in private to bolster Biden. And there is some reporting to back this up. As Politico reports, “Some of [Obama’s] aides now concede that behind the scenes Obama played a role in nudging things in Biden’s direction at the crucial moment when the Biden team was organizing former candidates to coalesce around Biden.”

Obama’s endorsement is a start—but it doesn’t go far enough. It’s unlikely that either Obama will convince all the Sanders supporters to line up behind Biden. But the best way to get as many as possible on board is to build on Obama’s endorsement by offering more concrete and specific concessions to the Sanders movement.

Biden’s campaign needs to keep acknowledging that he needs the Sanders movement. It’s not enough to say they have the same goals as Sanders supporters. He also needs to explain in detail how his administration will implement these promises. That includes naming the progressives who will staff his White House.

The sooner Biden fleshes out his offer to Sanders supporters the better. There is a core of “Bernie or Bust” dead-enders who can’t be won, but many more Sanders supporters can be brought into the fold. They still have the potential to be the most energized part of the party. Now is the time to consolidate the Democratic Party, so it can direct its energy towards defeating Trump.