Progressives want to extract a concession from former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE with his pick for a running mate after they failed to push one of their own to the top of the Democratic 2020 ticket.

Biden, a centrist who bested more liberal rivals such as Sens. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.) on his way to becoming the presumptive nominee, has already vowed to tap a woman as his vice presidential pick. But he will still have to traverse a political minefield to find a No. 2 who will most effectively expand his appeal and win over skeptical liberals.

Several advocacy groups told The Hill that tapping a progressive to share the ticket would go a long way in signaling to the party's left flank that he takes them — and their policies — seriously.

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“Joe Biden absolutely has to pick a progressive champion as his VP pick. He has to unify the party, and that’s the key,” Charles Chamberlain, head of Democracy for America, told The Hill. “What we saw during the primary is we saw that we have two major factions of this party, the corporate wing, more establishment Democrats, and there is progressive, ascendant left. And he absolutely has to choose from that progressive left to unify the party.”

Speculation over Biden’s pick for a running mate kicked into hyper drive last week after Sanders’s withdrawal made him the last man standing. But he will still have to recover after a divisive primary that exposed intraparty fissures.

The former vice president has consistently touted a moderate platform and campaigned on a return to normality, a stance anathema for progressives who demanded a “political revolution” to push the nation forward.

And while Biden has rejected leftwing policies including “Medicare for All” and the Green New Deal, liberal groups say the right running mate could still signal that he is willing to take progressive proposals with him into the Oval Office.

“Running with a vice presidential nominee and the eventual vice president, that person matters in an administration. They bring with them a base, they bring with them followers and supporters, they bring with them a certain set of values and a particular agenda that would have to be taken seriously,” said Maurice Mitchell, the national director of the Working Families Party.

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The obstacles Biden faces in shoring up his left flank have been made apparent since he became the de facto standard-bearer. Some liberal voters are still smarting over the defeats of Sanders and Warren, and a coalition of eight progressive groups sent the former vice president a letter with a list of policies they demand he adopt if he wants their support.

Prominent Sanders supporters have already vowed to withhold their support for now, citing Biden's reluctance to adopt key progressive platforms. Briahna Joy Gray, who was Sanders’s national spokeswoman, emphatically declared this week: "I don't endorse Joe Biden."

Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat Lawmakers fear voter backlash over failure to reach COVID-19 relief deal Why Democrats must confront extreme left wing incitement to violence MORE (D-N.Y.), a Sanders backer and one of the most prominent progressives in the nation, has also appeared tepid about a Biden endorsement, only saying "we'll see" during a web event this week when asked about declaring support for the former vice president.

“We’re having conversations with Biden’s team, and trying to figure out what some of these policy conversations will look like. I would love to see the vice president clarify and deepen his policy stances on certain issues," Ocasio-Cortez said, though she added, "I think it’s incredibly important we support the Democratic nominee in November."

Biden has offered preliminary olive branches to progressives, proposing dropping the age to qualify for Medicare from 65 to 60 and unveiling a plan to cancel some student debt.

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But while progressives told The Hill they appreciate Biden’s “first steps,” those proposals have not tempered their desire to see one of their own by his side.

“It’s both important for Vice President Biden to move on the issues, but also for him to surround himself with people who do have a track record of championing some of these issues that progressives care about deeply,” said Alliance for Youth Action executive director Sarah Audelo, whose group signed onto the letter to Biden.

Biden could pick from a number of progressive women to serve as his VP. Among the most prominent contenders who have been floated are Warren and Stacey Abrams, the former Georgia gubernatorial candidate and state House minority leader. Both have openly expressed interest in the role, with Abrams saying she would be an “excellent” running mate for Biden and Warren confirming that she would accept an offer to be his No. 2.

“I think Stacey Abrams in Georgia would fill a really important role to the ticket, it would bring a southerner from a very flippable state that also has flippable Senate races in it. She is very much loved by progressives, she would bring a lot to the ticket. Elizabeth Warren, fantastic, would definitely help unite the Bernie and Warren wing with the Biden wing,” Chamberlain said.

While Biden has expressed admiration for several progressive contenders, he has also indicated that he needs a running mate with whom he’s “simpatico,” drawing early warnings from liberals that a centrist pick could dampen enthusiasm for his candidacy and make his path to victory more difficult.

“Doubling down on the same as what you are doesn’t broaden your base, it doesn’t broaden your support,” Chamberlain said. “Does Joe Biden want a landslide victory because he’s unified the Democratic Party by putting the two most powerful constituencies together, or is he going to double down on the side of the party that won the nomination and alienate a critical part of his base?”

Biden said at a fundraiser earlier this month his campaign will announce a committee in the middle of April to oversee the vice presidential vetting process, and a campaign official told The Hill there are no other details “available at this time” regarding a pick.