WASHINGTON — Joe Biden is winning the delegate race over Bernie Sanders. Now comes the hard part — winning over Sanders’ supporters.

“Bernie brothers,” as Biden himself called them at a fundraiser last week, are known for their loyalty to the Vermont senator and their defections in 2016 to Donald Trump and third-party candidates may have contributed to Hillary Clinton’s loss.

At the end of their long, bitter primary, Clinton put the onus on Sanders to bring his backers into the fold. To avoid a repeat of four years ago, Biden will likely have to be more proactive and not count on Sanders to do the work unifying the party for him — even if that means the former vice president will have to turn the other cheek to ongoing attacks and rein in his own supporters’ desire to gloat or to speed Sanders’ exit.

Jennifer Palmieri, who was Clinton’s communications director four years ago, said Biden needs to give Sanders’ campaign space, pay homage to the movement it built, and recognize that attempts to strong-arm the candidate or his supporters will most likely backfire.

“You can’t force Bernie Sanders supporters to do anything. Even Bernie Sanders can’t force Bernie Sanders supporters to do anything,” she said. “Letting Sanders determine what’s best for his people is probably ultimately what’s best for Biden, so long as Sanders supporters are not echoing Trump’s arguments.”

“Outside pressure, forcing Sanders out of the race, is, I think, a big mistake,” she added. “Either Bernie Sanders is going to want to help the Democratic nominee or not. I believe that he does, so then you’ve got to give that campaign the room to do this the best way they can.”

Biden got off on the right foot, she said, by extending the olive branch in a speech after his victories on Tuesday.

“I want to thank Bernie Sanders and his supporters for their tireless passion,” Biden said. “We share a common goal, and together we’ll defeat Donald Trump.”

Sanders plans to stay in the race through at least Sunday’s debate, the first one-on-one with Biden, when it will become clear how aggressively Sanders plans to go after his rival.

Still, in a sign of how difficult this will be, some Sanders allies chafed at that sentiment, saying it’s too soon to talk about the Vermont senator dropping out since voters in more than half the states have not yet voted.

“It’s premature and getting off on the wrong foot to stand there and welcome Bernie supporters before Bernie has made a decision about whether to proceed or not,” said Jonathan Tasini, a progressive activist and former Sanders surrogate. “I really thought it was tone deaf and actually probably hurt as the first outreach to Bernie Sanders supporters.”

Tasini said Biden will need to go beyond rhetoric, adding, “The price of unity has to be extracting some very specific concessions on big issues that speak to what Bernie supporters want.”

But some Biden supporters say Sanders is no position to make any demands. South Carolina Rep. James Clyburn went so far as to call on the Democratic National Committee to “shut this primary down” and cancel future debates.

But that attitude led to walkouts of Sanders supporters at the 2016 convention and thousands of Sanders supporters staying home or voting for Trump.

And even if Biden ends up easily winning the party’s nomination, it will be because of his perceived ability to beat Trump more than his policy agenda or anything else.

“Look, I view myself as a bridge, not as anything else,” Biden said Monday at an event with Sens. Kamala Harris and Cory Booker, both former rivals for the nomination who have endorsed Biden. “There’s an entire generation of leaders you saw stand behind me. They are the future of this country.”

Biden is still missing a key pillar of the vaunted Obama coalition that Sanders won — young people — and Sanders also beat him among Latinos in the West.

Young voters are a large and growing share of the electorate and are more likely to be progressive than previous generations, but are also more likely to not affiliate themselves with either party, making winning them over “the ‘Manhattan Project’ both for the Biden campaign and the party,” as former Obama campaign manager David Plouffe said on MSNBC Wednesday.

In Michigan, for instance, a state Biden otherwise won handily, the former vice president lost 3 out of 4 voters aged 18-29. And he lost 30- to 44-year-olds, 54 percent to 42 percent.

Four years ago, after it became clear Sanders had no path to winning the Democratic nomination, his campaign shifted its focus to influencing the Democratic Party platform and rules.

Working into the wee hours in a hotel ballroom in Orlando, Florida, Sanders and Clinton officials hammered out compromises on everything from the minimum wage to abortion.

In the end, Sanders ended up getting almost everything he wanted, which means the low-hanging fruit has already been picked and Biden may need to make concessions on bigger, thornier issues.

“We need a vision for this country and for our party. And if it’s not ‘Medicare for All,’ what is it? If it’s not a Green New Deal, what is it?” Rep. Alexandria Ocasio-Cortez, D-N.Y., told reporters on Wednesday. “There are real electorates that Biden has lost, and I think it’s important for the sake of unity that we get real commitments on issues.”

Sanders seemed to open the first round of negotiations in public during his remarks Wednesday when he notably refrained from directly criticizing Biden and instead ticked through a litany of policy questions that he intends to ask Biden about at Sunday’s debate, including student debt, mass incarceration and immigration.

“On Sunday, I very much look forward to the debate in Arizona with my friend, Joe Biden. And let me be very frank as to the questions that I will be asking Joe,” Sanders said.

Mainstream Democrats are sure to worry that anything that extends the primary will only end up hurting Biden and helping Trump. And Sanders allies acknowledge there are some members of his coalition that will never come along with Biden, no matter how many progressive activists the ex-VP breaks bread with between now and November.

But Ana Maria Archila, the co-executive director of the progressive group Center for Popular Democracy, which backed Sanders, said Biden’s electability can only be enhanced by systematically courting progressives.

“I don’t see how the party is going to be able to defeat a movement candidate — because Donald Trump is a movement candidate — without a movement candidate,” Archila said. “So Joe Biden is going to have to do the work.”