The simmering animosity between former secretary of state Hillary Clinton and Senator Bernie Sanders boiled over on the debate stage, casting the first serious doubts over whether the Democrats will be able to unite behind the eventual nominee for the US presidency.
For months, the chaos within the Republican Party has obscured any questions over unity among the Democrats, but a testy, aggressive debate in Brooklyn on Thursday brought those problems to the fore.
The two rivals hit each other hard on a broad range of issues in an ill-tempered affair that had them interrupting each other, smirking and making snarky remarks.
One prolonged shouting match prompted the CNN moderator to chime in: "If you're both screaming at each other, the viewers won't be able to hear either of you."
A raucous, partisan audience added to the heated atmosphere. They cheered loudly, but also booed as the candidates were speaking. A group of Sanders supporters took to chanting "Bernie! Bernie!" towards the end of the debate.
I don't believe that that is the kind of judgment we need to be the kind of president we need.
SENATOR BERNIE SANDERS, on Mrs Clinton's support of the war in Iraq and numerous free trade deals and her decision to take campaign contributions from large corporations.
TALK IS EASY
It's easy to diagnose the problem. It's harder to do something about the problem.
FORMER SECRETARY OF STATE HILLARY CLINTON, taking a jibe at Mr Sanders.
The tone was set from the get-go. Mr Sanders started by backing away from his controversial assertion that Mrs Clinton is not qualified to be president, yet questioning if she has the judgment to hold the country's highest office. Pointing to her support of the war in Iraq and numerous free trade deals, as well as her decision to take campaign contributions from large corporations, he said: "I don't believe that that is the kind of judgement we need to be the kind of president we need."
Mrs Clinton hit back by citing an interview Mr Sanders did with the New York Daily News in which he failed to back up his campaign messages with specific policy ideas.
But for all the intensity, neither candidate landed any killer blows.
University of Michigan director of debate Aaron Kall said that while Mr Sanders had his best session to date, "nothing was done to alter the fundamentals of the race or likely impact voting in New York".
Mrs Clinton is favoured to win Tuesday's crucial New York primary, with 247 delegates up for grabs.
In fact, some of the best moments of the night belonged to her.
When her rival equivocated on a question about when Mrs Clinton allowed her decisions to be swayed by donations from Wall Street, she was quick to jump in. "He cannot come up with any example, because there is no example," she said to loud applause.
Later, during an exchange on tackling climate change, she took a jibe at him, saying: "It's easy to diagnose the problem. It's harder to do something about the problem."
The line underscored one of the sharpest differences in governance style between the two. Mr Sanders has campaigned as the dreamer with big ideas pushing for a political revolution. Mrs Clinton, meanwhile, is casting herself as the pragmatist.
That distinction was clear on nearly every issue. Though both appeared to agree on the goals for minimum wage, universal healthcare and climate change, they disagreed on how to get there. In each case, Mrs Clinton pitched incremental change while Mr Sanders pushed for a revolution. "We have got to understand that in America, we should be thinking big, not small," he said.
The differences, accompanied by the most combative Democratic Party debate to date, quickly prompted discussion of whether the party might splinter.
Associate Professor Julia Azari of Marquette University wrote on poll analysis site Fivethirtyeight that it was too soon to predict a crack-up: "Parties can contend with different factions, provided there's some common ground. The Republicans are dealing with different candidates who claim to carry the true vision of conservatism - and one insurgent who's challenged them all. The Sanders candidacy has been unexpected, but it's premature to expect that it's a signal of party weakness."