Story highlights Julian Zelizer: Clinton and Sanders played out epic battle in Brooklyn, with each coming out of distinct, longstanding Democratic tradition

He says Sanders needed to show he understood policy; Clinton needed to show heart, not just mind. She succeeded better than he

Brooklyn, New York (CNN) Brooklyn was raucous Thursday night. The two Democratic presidential candidates went head to head for their New York debate in the Duggal Greenhouse in Brooklyn -- and the atmosphere was, to say the least, tense.

It was impossible to miss the animosity that exits between the supporters of Hillary Clinton and Bernie Sanders: they have become deeply engaged in this epic battle between two very different voices within the Democratic Party, each of whom comes out of a very different, longstanding tradition.

Sanders is the warrior of the left who taps into the spirit of Ted Kennedy, Eugene McCarthy and other politicians who have argued that change starts from the bottom up and leaders need to be adamant about principle. And Clinton is the champion of Washington-based pragmatic liberalism who -- like Lyndon Johnson, Hubert Humphrey, Jimmy Carter, Bill Clinton and to a certain extent Barack Obama -- believes that Democrats need to make adjustments in order to survive within a very conservative political world.

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Tonight each campaign had a very different challenge as they arrived in Brooklyn ready to defend their approach. Sanders needed to assure voters that he has a strong grasp of the details of public policy and he needed to prove that he is still committed to the kind of campaign he promised when he once told the public he didn't care about Hillary Clinton's "damn emails."

For Clinton, the challenge was the same that she has faced from the very start of the campaign: the need to inspire voters and win over their hearts, not just their minds. Beyond the high decibel attacks each would end up making on the other, these were the challenges that were on the table when the candidates entered the room in Brooklyn.

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