Biden’s strategy capitalizes on Clinton's successful effort to brand Sanders as more unlikable than Biden among women, who favored the former vice president by 12 percentage points during his 10-state romp Super Tuesday, according to exit poll averages. With the averages showing Sanders lost black voters by even more — 34 points — the Biden campaign also tangentially brought up race in bashing the Vermont senator.

“We can’t divide this party like we did last time to allow Trump to get reelected,” Biden campaign co-chairman Rep. Cedric Richmond, a member of the Congressional Black Caucus from Louisiana, told reporters on a conference call Wednesday.

“The fact that Senator Sanders is on TV today with negative attack ads and consistently saying the establishment is teaming against him, no, African American voters have made a conscious decision that we fought and we earned civil rights through blood, sweat and tears,” Richmond said. “And so as he rails against the establishment, I did not know that African Americans in the South were considered part of the establishment.”

Sanders’ campaign did not respond for a comment.

The sharp turn in the primary marked the kickoff of the de facto two-man race between Sanders and Biden. The former is waging a campaign about policy. The latter is focusing on personality. Though Biden remains nominally ahead in the race for delegates needed to win the nomination at the party’s convention in June, he does not have a commanding lead and tensions are escalating.

In a sign of his campaign’s struggles, Sanders finally gave in to advisers who had been pushing for months to go negative on Biden, releasing two ads criticizing him. Sanders then personally laid into Biden at a news conference in Vermont Wednesday.

“Joe is going to have to explain to the people, the union workers in the Midwest, why he’s supported disastrous trade agreements,” Sanders said. “Joe is going to have to explain to the American people why he voted for a Wall Street bailout, something that I vigorously opposed.”

Sanders repeated the phrase “Joe is going to have to explain” three more times concerning Biden’s positions on authorizing the Iraq War, supporting a “a disastrous bankruptcy bill which benefited the credit card companies” and advocating cuts to Social Security, Medicare and Medicaid.

But Biden, who has addressed many of the criticisms over the months in debates, town halls and media interviews, has little interest in fighting on the terrain picked by Sanders.

Now flush with cash — Biden advisers said the campaign raised $15 million in three days — the campaign is better positioned to go head-to-head on paid messaging, including television ads that were already running in the next batch of primary states.

“The Biden strategy is to make Bernie pay as steep a price as possible among electability-minded voters to whom Bernie’s attacks would be off-putting because it reminds them of" the divisions in 2016, said Brian Fallon, a former spokesman for Clinton. He predicted that Sanders would repeat his primary tactics of four years ago by going negative on Biden.

Hours later, Sanders did just that. And Biden’s deputy campaign manager, Kate Bedingfield, was quick to remind reporters of the past.

“We’ve seen unfortunately what kind of campaigns Bernie Sanders runs,” Bedingfield said. “We saw the impact that it had in 2016.”

Biden supporters, from donors to surrogates, huddled Wednesday to devise how to handle what they expect will be an onslaught of negative attacks, not just from Sanders, but from Republicans who never quite counted out the former vice president. Already on Wednesday, GOP members of Congress started up talk of investigations of Biden that would benefit Trump.

Biden advisers note the attacks are nothing new and that over the past year, even amid a barrage of attempted takedowns, Biden showed resiliency, ultimately emerging the sole surviving moderate in what began as a vast primary field. They also say the sustained attention from Republicans is further evidence of the threat Trump believes Biden poses in a general election.

Still, Biden’s campaign has had its share of criticisms even from supporters, most notably Rep. Jim Clyburn, who complained of a deficient field organization and poor fundraising. On Wednesday, Biden’s media organization attempted to keep a tight ship on messaging, according to emails the campaign sent to surrogates that was obtained by POLITICO.

“We wanted to put in a quick ask as we’re hearing a lot from bookers, who I’m sure will also start reaching out independently. We’re asking that you do not do any TV hits tonight talking about results/the campaign,” an email from the campaign said. “We're trying to keep a tight, united front, as we centralize messaging from our staff here.”

Biden’s camp is pushing forward with a strategy whose success the past four days is indisputable: framing him as the momentum candidate. To that end, staffers continue lining up congressional and local endorsements in upcoming states, work that’s made easier by Biden’s against-the-odds victories across the Super Tuesday map.

Sanders, too, is making an electability argument, saying Biden will ultimately suffer in a matchup against Trump over issues like trade.

Sanders said an issues debate is what voters want.

“I do not want this campaign to degenerate into a Trump-type effort where we're attacking each other, where it's personal attacks,” Sanders said, emphasizing he wants to debate substance.

In a separate email to supporters, Biden’s campaign noted his strength in the suburbs and swing districts, an argument Biden has attempted to underscore for his candidacy for months on the trail. Now the campaign has data points to back it up.

“Biden won people of all backgrounds. He did very well in the suburbs and swing districts,” the email read. “He won in the places that Democrats won in 2018.”

Laura Barron-Lopez contributed to this report.