Held in prime time and on a weeknight -- unlike most Democratic debates -- the CNN-sponsored event was the highest-profile opportunity for both campaigns to make their final arguments before Tuesday's crucial vote in a state where both contenders have strong roots. Clinton is looking to New York to solidify her role as front-runner, while a strong showing -- or a victory -- for Sanders would deal a significant blow to her confidence and bolster his campaign's argument that the party's so-called super delegates should switch their allegiance to him.
The Democratic candidates took sharp aim at one another almost immediately over a series of issues, including the Iraq War, Wall Street and questions about judgment and qualifications to be president.
Both contenders shifted back and forth between offense and defense, a sharp departure from the calmer tone of the party's earlier debates.
Sanders began by explaining a recent comment he made on the campaign trail in which he suggested Clinton was "unqualified" to be president.
"Does Secretary Clinton have the experience and intelligence to be president? Of course she does. But I do question her judgment," Sanders said, pointing to her Senate vote for war in Iraq, her willingness for her campaign to benefit from millions of dollars spent on her behalf by super PACs and her relationship to Wall Street.
"Senator Sanders did call me unqualified. I've been called a lot of things in my life. That was a first," Clinton responded, pointing to the fact that she was elected twice to the Senate and chosen as secretary of state.
Clinton then counter-attacked, citing an interview Sanders gave to the New York Daily News in which he struggled to provide specifics about his plans for breaking up banks and other issues.
"Talk about judgment and talk about the kinds of difficulty he had answering questions, including his core issues," Clinton said.
The sparring continued throughout the night, so much so that at one point, CNN debate moderator Wolf Blitzer moved to break up the fighters.
"If you're both screaming at each other, the viewers won't be able to hear either of you," Blitzer said after Clinton and Sanders spent several seconds talking over each other.
2. Clinton further than ever from a general election pivot
Just a month ago, Clinton appeared poised and eager to pivot to the general election and start building a case against a Republican nominee.
But Thursday's attacks on Sanders showed that her campaign realizes that she has to turn all of her attention to her left flank.
At previous debates, she seemed to spend as much time talking about Republicans as Sanders. Not in Brooklyn.
Clinton arrived at Thursday's debate with policy knives sharpened and ready. An hour before the contest, her campaign released a memo outlining the case she would make against Sanders and repeatedly released briefings throughout the night that both tried to defend her from attacks. At every turn possible, she criticized him on gun regulation, the release date of his tax returns, how he would break up big banks, provide Medicare for all and other issues.
Her campaign knows that next week's primary could be a major opportunity for her to stop Sanders' fast-growing momentum -- or else. With time running out before Democrats go the polls, Clinton's aggressive tactics suggest that she knows she needs to start making some of the blows against Sanders count.
3. 'Think big' or get things done?
A key difference in Clinton's and Sanders' approaches to governing was on full display during the debate over climate change.
Clinton is a politician who is content with incremental change, seeing it as the most realistic -- if not the only -- way to achieve her goals. The Vermont senator wants sweeping change, and believes the nation's problems are too big for singles and doubles. He wants to swing for the fences.
Their debate over climate change, in particular, highlighted their contrasting philosophies.
"Incremental steps are not enough," Sanders said after Clinton knocked him for faulting the recent international Paris agreement on climate change.
Clinton fired back, "I don't take a back seat to your legislation that you have introduced that you have been unable to get passed."
This is the heart of their differences: To Sanders, the Paris climate agreement
does not go far enough, and therefore isn't good enough. To Clinton, the Paris agreement was the best deal possible.
4. Clinton won't give on transparency
Once again, Clinton came under fire for keeping the content of her highly paid speeches to financial firms under wraps after she left the State Department.
Sanders has hammered her for refusing to release transcripts of remarks she made to companies like Goldman Sachs, gigs that have earned her millions.
"Why not just release the transcripts and put this whole issue to bed?" CNN co-moderator Dana Bash asked Clinton.
Clinton tried to use the question to criticize Sanders over financial reform, but Bash continued to press the issue. Clinton said she only would release transcripts if Republicans did the same.
"There are certain expectations when you run for president. This is a new one. And I've said, if everybody agrees to do it -- because there are speeches for money on the other side. I know that," Clinton said, and again pivoted to Sanders by criticizing him for not yet releasing his tax returns.
Sanders responded that he planned to release a year of tax returns Friday and would unveil more soon, which turned the attention back on Clintons' unwillingness to release the transcripts.
The exchange highlighted Clinton's struggle with transparency--whether it's over her State Department emails or what she tells Wall Street behind closed doors.
5. One place where Sanders and Clinton agreed: Regrets
Clinton and Sanders both acknowledged that the aggressive measures they championed in the 1990s to fight crime have proven to have disproportionately negative affects on African American communities, a fact for which she apologized Thursday.
When asked if the move was a "net positive" in the fight against crime, Clinton defended the good intentions of the measure, saying "it had some positive aspects to it," but conceded that as new information has come to light about the adverse affects of policy, so should approaches to problems.
"If we were to have the balance sheet on one side there are some positive actions and changes. On the other side there were decisions that were made that now we must revisit and we have to correct," she said, adding later: "I'm sorry for the consequences that were unintended and that have had a very unfortunate impact on people's lives. I've seen the results of what has happened in families and in communities. That's why I chose to make my very first speech a year ago on this issue... because I want to focus the attention of our country and to make the changes we need to make."
In a rare moment of agreement during an otherwise contentious night, Sanders also expressed regret for some of the outcomes of the bill.
"Much of what Secretary Clinton said was right. We had a crime bill. I voted for it. It had the Violence Against Women Act in it," he said. "But where we are today is we have a broken criminal justice system. We have more people in jail than any other country on Earth. And in my view, what we have got to do is rethink the system from the bottom on up. And that means, for a start -- and we don't talk about this. The media doesn't talk about it -- you got 51 percent of African-American kids today who graduated high school who are unemployed or underemployed. You know what I think? Maybe we invest in jobs and education for those kids, not jails and incarceration."
6. But no apology from Sanders on guns
The apologies largely stopped there, however, as Clinton took on Sanders over gun policy.
When Blitzer said that a parent of a victim of the 2012 Sandy Hook Elementary School shootings had called on Sanders to apologize for opposing a measure that would allow victims of gun violence to sue firearms companies, he declined.
"I voted against this gun liability law because I was concerned that in rural areas all over this country, if a gun shop owner sells a weapon legally to somebody, and that person then goes out and kills somebody, I don't believe it is appropriate that that gun shop owner who just sold a legal weapon to be held accountable and be sued," Sanders said. "But, what I do believe is when gun shop owners and others knowingly are selling weapons to people who should not have them -- somebody walks in. They want thousands of rounds of ammunition, or they want a whole lot of guns, yes, that gun shop owner or that gun manufacturer should be held liable."
"So, Senator, do you owe the Sandy Hook families an apology?" Blitzer asked.
"No, I don't think I owe them an apology. They are in court today, and actually they won a preliminary decision today. They have the right to sue, and I support them and anyone else who wants the right to sue."