(CNN)After Joe Biden's stunning comeback on Super Tuesday, the Democratic Party is once again facing the nightmare of 2016: a collision course between its establishment candidate and Bernie Sanders that could leave the party fractured and weakened come November.
Four years ago, some of Sen. Sanders' most ardent supporters -- including many young voters -- stayed home on Election Day, rejecting Hillary Clinton as their nominee. This campaign cycle, many of the Vermont independent's supporters are still deeply distrustful of a Democratic establishment that they believe wrested the nomination away from Sanders.
Though all sides claim they will be united in working to defeat President Donald Trump, the brewing Biden-Sanders battle could run even hotter than the Clinton-Sanders clash, because of Biden's and Sanders' dueling strengths, which have divided the party along lines of race and age.
There was no brilliant shift in strategy that lifted Biden's campaign from near-death before South Carolina to victory there and then to resurgence on Super Tuesday. Instead, he was thrust back into front-runner contention by a series of weighty endorsements, starting with US Rep. Jim Clyburn in South Carolina and former Virginia Gov. Terry McAuliffe and swiftly followed by three former primary rivals: Pete Buttigieg, Amy Klobuchar and Beto O'Rourke. It was the sign that undecided and skittish Democratic voters clearly had been looking for.
The big endorsements for Biden kept coming after Tuesday night's showing. He was anointed by another force that Sanders supporters loathe when former New York Mayor Michael Bloomberg on Wednesday threw his backing behind the former vice president -- an embrace that could mean millions of dollars continue to be spent on ensuring that Sanders is not the Democratic nominee. Former Michigan Gov. Jennifer Granholm also backed the former vice president, giving him a boost in a key state that will vote next Tuesday, along with former New Orleans Mayor Mitch Landrieu.
In an echo of 2016, the undertone of suspicion among Sanders supporters resurfaced Wednesday on social media with posts carrying the hashtag #RiggedPrimary and #RiggedDNC. In recent interviews at Sanders rallies, his supporters have openly expressed their fear that Democratic Party leaders will "steal" the nomination from him.
Sanders himself outlined an us-vs.-them construct for the campaign at a news conference Wednesday in Burlington, Vermont, where he listed the forces stacked against him, including Wall Street and "the entire corporate establishment."
"There has been never a campaign in recent history which has taken on the entire political establishment, and that is an establishment which is working frantically to try to defeat us," he said.
"There has not been a campaign that is trying to deal with the kind of venom that we're seeing from some in the corporate media," Sanders added. "This campaign has been compared to the coronavirus on television. We have been described as the Nazi army marching across France, etc., etc. As we come into the last several months of this campaign, what I hope very much is that what we can focus on is an issue-oriented campaign, which deals with the concerns of the American people."
When asked about Bloomberg, Sanders said he had "no animus" toward the former mayor, "but this just confirms exactly what I said. It's what the media has been talking about for months. How do we stop Bernie Sanders? How do we stop a movement of working people and low-income people? How do we stop a multi-generational, multi-racial movement, which is standing up for justice?"
Biden's aides, in turn, were sharply critical of what they view as divisive tactics by the Sanders team in upcoming states, including a new ad critiquing the former vice president's advocacy for freezing Social Security spending in 1995 (an argument the then-senator from Delaware made when supporting a balanced budget amendment). Another ad touted Sanders by using past praise from former President Barack Obama.
"We've seen, unfortunately, what kind of campaigns Bernie Sanders runs. We saw the impact that it had in 2016," Biden deputy campaign manager Kate Bedingfield told reporters.
US Rep. Cedric Richmond, a co-chair of Biden's campaign, also took issue with Sanders' characterization of Biden's victories Tuesday night, pointing to his huge margins among black voters.
"I just did not know that African Americans in the South were considered part of the establishment," the Louisiana Democrat said.
In a quest for unity, Democratic leaders like Obama and House Speaker Nancy Pelosi have studiously stayed out of the fight. But Clinton, whose disdain for Sanders has continued to draw headlines, was blunt about where her preferences stand in a new Hulu documentary series, "Hillary," that premiered at Sundance.
"Nobody likes him," she said of Sanders. "Nobody wants to work with him."
Among Democrats more broadly, Jen Psaki, a former Obama adviser who's now a CNN contributor, pointed out that since Clinton's defeat in 2016, there has been a much greater recognition of the power and importance of Sanders' coalition, which is why Biden has labored to strike a tone of inclusivity.
"There wasn't the same fear of (Trump) going into the White House that exists now, and that will be a big motivator that it was not four years ago," Psaki said. "There was a lack of understanding of the power of Bernie Sanders' support and the loyalty of his support. And there probably wasn't enough seriousness taken -- the need to reach out to people who felt disaffected, who didn't feel heard, who felt that their movement wasn't taken seriously and was undervalued when it shouldn't have been."
"On the flip side, the lesson has not been learned on the Bernie Sanders side, in my view. This is about addition, not subtraction," Psaki said. "And you're not going to win by alienating large swaths of Democrats out there -- people who don't feel like they want to be part of Bernie Sanders' movement, they just want to defeat Donald Trump and they want their health care protected."
A divide by age and race
The delegate count remained close -- Biden led with 509 delegates to Sanders' 449 by Wednesday evening -- but the fault lines beneath those numbers were striking, underscoring how much work both of them have ahead as they work toward becoming the party's nominee.
African American voters powered Biden's victories in the South. In Alabama, for example, where nearly half the electorate was black, 72% of black voters supported Biden while 10% backed Sanders. (Biden won 56% of black voters overall, compared with 17% who were behind Sanders and 15% who backed Bloomberg, according to a CNN analysis of the exit polls that were conducted across 12 states on Super Tuesday.)
As they did in the early state of Nevada, Latinos favored Sanders by 44%, to 25% for Biden. Biden was also favored by older voters, while Sanders maintained his huge edge among younger voters. The former vice president carried 46% of voters 65 and older, while 14% of them supported Sanders, according to CNN's analysis of the exit polls. Sanders once again dominated among voters under 30, with 58% of them backing his campaign, compared with 14% for Biden and 13% for Sen. Elizabeth Warren of Massachusetts, according to CNN's analysis.
Sanders' claim that he can expand the Democratic coalition by bringing out a younger, more diverse group of voters, while also activating working-class voters who have opted out of the system, so far has not come to fruition. In states like North Carolina and Massachusetts, Sanders did not do as well with non-college-educated white voters as he did four years ago. His loss in Minnesota, another state with a large working-class population where he'd hoped to do well, was also notable.
"Bernie Sanders needs to grow his support significantly to win the nomination and that begins with the African American community. If he doesn't do much better with those voters, his chances of being the nominee are awfully close to zero," said Dan Pfeiffer, a former senior adviser to Obama. "That alone will not be enough. To date, his campaign has at times given off a 'my way or the highway vibe' that has put a ceiling on his support among large swaths of the party."
The senator acknowledged on Wednesday that expanding the Democratic coalition has been hard for his campaign. As of 4 p.m. ET on Wednesday, Biden was leading Sanders in the popular vote by 900,000 ballots, depriving the senator of the argument about his people-powered campaign that he had used after losing the delegate count in Iowa to Buttigieg (but leading in the raw vote totals).
"We're making some progress, but historically everybody knows that young people do not vote in the kind of numbers that older people vote in," Sanders said. "I think that will change in the general election, but I am honest. ... We have not done as well in bringing young people into the political process. It is not easy."
He framed the newly shaped race in stark terms. The question for voters, he said, "is which side are you on?"
CNN's Eric Bradner, David Chalian and Jennifer Agiesta contributed to this story.