It's clear – Joe Biden has his work cut out for him as he tries to unite the Democratic Party.

The day after delivering a more aggressive than expected debate performance against the former vice president and current presidential primary front-runner, Sen. Bernie Sanders vowed that “this primary is far from over.”

In an email to supporters, the progressive firebrand from Vermont took aim at his rival as he spotlighted that Biden “has argued that we have to cut Social Security, voted for the war in Iraq, supported the Hyde Amendment that restricts poor women’s access to abortion, has threatened to veto legislation that will guarantee health care as a right, led the fight for the disastrous bankruptcy bill.”

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Sanders also targeted Biden on Twitter, writing “you need to take on Wall Street, the drug companies, the insurance companies, and the fossil fuel industry. You don’t take campaign contributions from them.”

The night before, during the debate, the self-declared democratic socialist repeatedly jabbed at Biden over his long record in Washington, his fundraising from corporations and his support from outside super PACs.

Sanders also turned down a symbolic policy olive branch from Biden, who in the lead-up to the debate embraced progressive policies pushed by Sanders on free college tuition and bankruptcy reform. A frustrated Biden said during the prime-time showdown that Sanders is “making it hard for me” to bridge the gap between them.

The deep policy divisions on display during and following the debate are, in turn, renewing fears that Democrats could experience a repeat of 2016 – when many Sanders supporters wouldn't vote for eventual nominee Hillary Clinton after a long and divisive primary battle. Democrats, desperate to defeat President Donald Trump in November’s general election, are trying to prevent heading into the general election with the progressive base of the party once again fractured.

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Since Biden’s sweeping victories in last Tuesday’s primaries, when he took a large lead over Sanders in the crucial race for convention delegates and cemented his front-runner status, #NeverBiden has started to trend on Twitter.

And Rep. Alexandria Ocasio-Cortez of New York – a top Sanders surrogate and progressive rock star – took to Twitter Sunday night to repeatedly bash Biden over what she seemed to frame as policy flip-flops. The tweet indicated lingering resistance on the left to embracing Biden's appeals to progressives.

“Don’t say you support a fracking ban when you don’t. Don’t say you didn’t write the bankruptcy bill when you were one of it’s biggest champions. Don’t say you supported ending Hyde a long time ago when you were just pressured into it last year,” the congresswoman tweeted. “This is basic. The truth matters.”

As he tries to keep the peace, a comment immediately after the debate from a Biden senior adviser may not have helped.

“It’s safe to say Vice President Biden showed up to a debate tonight and for two hours graciously [dealt] with a kind of protester who often shows up at campaign events, on live television,” Anita Dunn told reporters on a conference call.

Others on the call worried about a drawn-out and contentious primary harming the party's general election goals.

The latest clashes have a leading Sanders supporter in New Hampshire worried.

“Yes. It will be work and I intend to work as hard as I can to unite the ticket, as I did in 2016,” former state Sen. Burt Cohen told Fox News. “It’s not going to be easy. I hope it’s not as hard as it was in 2016.”

And Cohen is not sure Biden’s up to the task of bringing the Sanders wing of the party into the fold.

“I worry about Joe Biden frankly,” Cohen said. “When I see him, he doesn’t have any charisma. He doesn’t get people excited.”

Adam Green, co-founder of the pro-Warren Progressive Change Campaign Committee, last week urged Sanders to stay in the race to debate Biden.

“He was more aggressive than I thought he was going to be but he was less effective at securing concrete commitments on progressive issues from Biden than I thought he would be given how aggressive he was,” noted Green, who along with the PCCC were major supporters of progressive Sen. Elizabeth Warren’s White House bid.

Asked if he’s concerned about a repeat of the 2016 Democratic divisions, Green told Fox News “not yet.”

“Especially in this new coronavirus crisis world, Democrats need to see how our nominee will campaign and sturdy they are and last night’s debate – even if it was a bit messy – was productive for both candidates to seem more presidential and more correct on policy than Trump. More of that so we have a tested candidate is good.”

Democratic strategist Chris Moyer disagreed with those predicting another chasm.

“It does not seem like Senator Sanders is taking the same approach as he did four years ago,” argued Moyer, a veteran of the 2016 Hillary Clinton presidential campaign as well as Sen. Cory Booker of New Jersey’s 2020 White House bid. “While many thought he might handle the debate differently, right now there are not broad concerns about a long, drawn-out primary that hurts our eventual nominee, which is quite clearly going to be Vice President Biden.”