The former press secretary for Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE’s (I-Vt.) presidential campaign said Monday that she would not endorse former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE, shortly after Sanders appeared in an online video to announce his support for his former rival in the general election against President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE.

Briahna Joy Gray, who was Sanders’s national spokeswoman until the campaign was suspended last week, has become a Twitter sensation in recent days for her searing attacks against Biden.

On Monday, just minutes after Sanders and Biden held a joint press conference to talk about how they would work together to defeat Trump, Gray responded on Twitter by saying she would not endorse Biden.

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With the utmost respect for Bernie Sanders, who is an incredible human being & a genuine inspiration, I don't endorse Joe Biden.



I supported Bernie Sanders because he backed ideas like #MedicareForAll, cancelling ALL student debt, & a wealth tax. Biden supports none of those. — Briahna Joy Gray (@briebriejoy) April 13, 2020

The tweet underscores the work Biden has ahead of him to win over some of Sanders’s most ardent supporters, who insist they will not turn out for Biden simply as an alternative to Trump.

In their joint press conference, Biden and Sanders acknowledged their policy differences, particularly on the issue of "Medicare for All," the signature issue energizing the left.

But Sanders said he would put aside his policy differences with Biden, and the two agreed to set up policy working committees to move the party toward achieving Sanders's progressive goals.

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In addition, Sanders urged his younger supporters to put aside their disappointment and called on them to work to ensure Trump is a one-term president.

Biden made a direct plea to Sanders’s young base of supporters, saying he would do whatever it takes to win their votes.

“To 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,” Biden said.

Polls show that Biden has more than 90 percent support among Democratic voters.

At the same time, there is a small but vocal contingent of influential left-wing Twitter and YouTube personalities who are deeply hostile toward Biden, and might become an issue for his campaign as the general election heats up.

Over the weekend, Gray detailed over Twitter why she would not support Biden.