Sen. Bernie Sanders (I-VT) may be endorsing Joe Biden for the Democrat nomination, but the political movement that powered the Vermont septuagenarian’s campaign is unready to do the same.

On Monday, shortly after Sanders threw his backing behind the former vice president, the senator’s one-time campaign spokeswoman, Briahna Joy Gray, announced that she was not following suit. Gray, who has made no effort to conceal disagreement with other high profile national Democrats, asserted she was unable to support Biden because of his unwillingness to adopt progressive stances on the economy and healthcare.

“With the utmost respect for Bernie Sanders, who is an incredible human being [and] a genuine inspiration, I don’t endorse Joe Biden,” she wrote on social media.

“I supported Bernie Sanders because he backed ideas like Medicare for All, cancelling ALL student debt, [and] a wealth tax,” Gray added. “Biden supports none of those.”

The ideological differences that Gray noted underscore the challenge that lays ahead for Biden. Since the former vice president routed his opposition on Super Tuesday to become the presumptive nominee, progressives aligned with Sanders began eyeing their options for the general election. Many seem to have found Biden, who has unequivocally come out against universal healthcare and has shown disdain for the “socialist” label, wanting.

Nor do such activists appear ready to come around any time soon. In recent weeks, as it became all but apparent that the former vice president would be the nominee, a small but vocal “Never Biden” community sprouted up on social media. Its members include former campaign operatives, sympathetic media pundits, and the types of average voters that made Sanders’ 2016 insurgent campaign possible.

Biden, for his part, has offered several olive branches to the disaffected Sanders supporters. Last month, the former vice president opted to adopt the bankruptcy proposal authored by his former rival, Sen. Elizabeth Warren (D-MA). The decision was nothing short of a full repudiation of the Bankruptcy Abuse Prevention and Consumer Protection Act of 2005, which Biden authored and progressives have long claimed was a giveaway to big banks. Biden’s shift on bankruptcy was followed up last week by an embrace of lowering the age of Medicaid eligibility to 60 and student loan forgiveness for individuals making below $125,000-per-year.

Those efforts, however, have not been succesful. More than a few members of Sanders’ movement have rejected the olive branches as not going far enough. Rep. Alexandria Ocasio-Cortez (D-NY), who endorsed the Vermont senator last year and campaigned for him ahead of the Iowa Caucuses, described Biden’s proposal as “almost an insult.”

“I think Hillary [Clinton] was looking at policies that lowered it to 50,” the congresswoman told The New York Times in an interview published Monday. “So we’re talking about a ‘progressive concession’ that is ten years worse than what the nominee had in 2016.”

Policy differences are not the only reason why many members of Sanders’ political movement remain uncommitted about backing Biden. Some, in particular, are concerned about new sexual assault allegation leveled against the former vice president by former staffer Tara Reade as disqualifying.

In fact, when Reade’s allegations first surfaced last month, a top Sanders supporter announced they believed the accusations were true. Others from the Never Biden community soon echoed the sentiment, with some even openly saying the allegations were disqualifying for the former vice president’s candidacy.

Hear the full story of Tara Reade, former Senate Aide to @JoeBiden, in her own voice, as she painfully describes being sexually assaulted by him in his office. She told this story to her trusted friends & relatives when it happened. #iBelieveTara. https://t.co/nSUGzsd2AT — Shaun King (@shaunking) March 25, 2020

All of this poses a major predicament. In 2016, President Donald Trump won the White House, in part, because disaffected Sanders supporters opted to stay home or vote third party. Decreased turnout for the Democrat ticket that year, coupled with Trump’s populist appeals to white working-class voters in the MidWest, led to a GOP victory.

A similar scenario could repeat itself this year. Exit polls from last month’s Michigan primary found that only two out of five Sanders supporters would be willing to back Biden in November, according to Politico. If those numbers hold on election day, it would spell disaster for Biden, especially as Michigan is considered a must-win if Democrats have any hope of retaking the White House.

Sanders, himself, appears to understand the predicament members of his one-time movement pose for Biden. On Monday, while endorsing the former vice president, the Vermont senator urged his supporters to fall in line behind the Democrat nominee.

“We must come together to defeat the most dangerous president in modern history,” Sanders said.