Former Vice President Joe Biden gained a coveted endorsement from former rival Sen. Bernie Sanders on Monday, but the question of whether his campaign's outreach efforts are enough to garner the support of the Democrats' progressive wing remains.

In a new interview published by The New York Times Monday, progressive Rep. Alexandria Ocasio-Cortez, D-N.Y., a staunch Sanders supporter and former campaign surrogate, said she had never spoken to Biden directly, but expressed a willingness potentially to campaign alongside him, depending on the direction that his campaign would take.

Ocasio-Cortez, who has criticized Biden in the past, said she planned to support the presumptive nominee in the general election but added that his campaign needed to address concerns by the party’s left wing. Though she told the publication she did not want to be "divisive," she said the process of unifying the party should not be easy, and made a distinction between supporting Biden and offering a full endorsement of his campaign.

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"The whole process of coming together should be uncomfortable for everyone involved—that's how you know it's working," she said. "If Biden is only doing things he's comfortable with, then it's not enough."

The Biden campaign declined to comment on the Times report.

In the last several weeks, the campaign has tried to unite more moderate Democrats and progressive voters, a group Biden has struggled to court. Aside from engaging younger Americans and advocates for race and justice issues, they said they're also focusing specifically on certain groups: legacy progressive organizations with extensive policy focuses, large national networks such as Planned Parenthood and new-movement progressive organizations using grassroots organizing to focus on social-justice issues -- groups including March for our Lives.

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Ahead of Sanders' surprise endorsement of Biden, spokesman Matt Hill told Fox News the Biden campaign was "continuously considering and evaluating additional policies that would build upon Vice President Biden's progressive agenda," adding that the campaign has been in contact with progressive leaders and organizations in the last month.

"The most important piece of engaging progressive leaders and groups is to ensure we're expanding and broadening our coalition to beat Donald Trump in November, and we are working with these leaders and groups to align on our efforts to do that. Defeating Trump is the universal commitment that we all share," he emphasized.

Last Thursday, one day after Sanders suspended his campaign, Biden released two new policy proposals mirroring Sanders' ideas to lower the Medicare eligibility age from 65 to 60 and forgive student debt for low-income and middle-class families.

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But, University of Houston political science professor Brandon Rottinghaus warns Sanders' full-throated endorsement might not be enough.

"This will partially help to unite the party from a symbolic point of view. From a substantive view, many in Sanders' camp won't be satisfied with just a surface-level endorsement. The hardcore Sanders supporters want substantive change that they're worried Joe Biden or another establishment candidate won't provide. If there's no substantive change that Bien provides, then Sanders' endorsement would mean very little," Rottinghaus said.

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Meanwhile, Biden, during a joint virtual event with Sanders on Monday, invoked the Vermont senator's slogan, "Not Me, Us," in an appeal to his supporters.

"I'm excited to do the work with you in the months and years ahead. You know, as you say, not me, us, not me, us, that's your phrase pal, and your supporters and I are going to make the same commitment... I tell your supporters I see you, I hear you. I understand the urgency of what it is that we have to get done in this country, and I hope you'll join us. The more the merrier," Biden said.