(CNN) I was fortunate to have the opportunity to watch Thursday's Democratic debate in Brooklyn live and in person. Although I have seen the other debates on television, there was something more visceral about being there.

What was most dramatic was not any particular point in the debate or any statement the candidates made, but the display of sheer and intense animosity that has developed between Hillary Clinton and Bernie Sanders supporters. With so much attention focused on the civil war taking place among Republicans, commentators may have missed the anger brewing among Democrats.

The tension was on display to everyone in the room. The candidates themselves barely looked at each other during the commercial breaks or when the contest ended.

Besides the tense words they exchanged, their body language made it clear there is now considerable fury at the very top of the campaign about how each has characterized the other and about the kind of politics each represents.

At one moment, CNN moderator Wolf Blitzer implored both of them to stop "screaming at each other" or else the "viewers won't be able to hear either of you. ..."

The tensions on the stage were nothing compared to the tension in the crowd. I sat next to one Sanders supporter who sat with his tattooed arms tightly crossed and head angrily shaking whenever Clinton said anything. In his mind it was clear there was nothing Clinton could do or say that was right.

Even when Clinton made a statement about the need to defend women's reproductive rights, something Sanders strongly supports, he and his friends wouldn't stand up. Throughout Clinton's comments, they yelled that she was bought by Wall Street or how she was a liar who refused to answer any question, "yes" or "no." One of the people in the group said, "I'm a Bernie Democrat. She's Trump light." They were not alone.

Throughout the room, Sanders supporters made it very clear they disliked Clinton almost as passionately as they dislike the GOP. When Clinton said the party will need to unite when the primaries were over, the man to my left said: "not for you. We just won't vote." His friends agreed.

Clinton supporters didn't feel much better about their opponents. Though not with the same vigor, they yelled when Sanders made statements, saying he was lying about her record or that he would not answer any question with specific statements. In their view, he was an idealist, a demagogue, a candidate who was making promises he could never keep.

They yelled at Sanders supporters to sit down when they cheered. At one unpleasant moment, a Clinton supporter sitting behind me mockingly yelled at three of the Sanders backers who were standing up and pumping their fists a slogan used by the Nazis, "Sieg Heil." The nasty comment even led his friend to suggest he calm down. "That's offensive," another Sanders supporter, who had been sitting quietly and calmly throughout the evening, turned to say.

The tensions are not unique to the debate audience in Brooklyn. A look at the online universe quickly reveals angry vitriol that is flying back and forth between the campaigns. The supporters on each side are reaching the point of demonizing the other, questioning their legitimacy and denying that there is any virtue on the other side. Write something that is slightly positive about either candidate (I have heard it from both sides) and you are instantly subject to a barrage of fury about your words.

This is a dangerous state for Democrats. It is logical and expected that passions run high during political campaigns. When primaries are competitive and last for a long time, and when candidates inspire passionate followers, fights tend to break out and feelings are hot.

But there does come a time when both sides need to unite or the party will pay the price. In 2008, when the anger between Obama and Clinton supporters ran high, the party was able to move beyond the division and focus their attention on the GOP once Obama received the nomination.

Most Clinton supporters joined the party fold, as did the Clintons, and mounted a united front against a danger they deemed much greater to the nation.

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There have been other times, however, when unity didn't come and the party suffered. Most famous was the political disaster of 1968 when the Democratic party had broken so far apart over issues like Vietnam that party leaders were unable to bring it together. Many supporters of Eugene McCarthy or Robert Kennedy couldn't stomach supporting Hubert Humphrey, part of an administration that brought the nation and the world the Vietnam War. The result was a Republican president, Richard Nixon.

Similar tensions, over different issues, played out in 1980 when supporters of Sen. Ted Kennedy could never find it in their heart to throw themselves behind the re-election of Jimmy Carter, who in their minds had abandoned the Democratic tradition.

The most vivid image from the Democratic convention, besides Kennedy's rousing speech, was when Carter had to literally scramble to find Kennedy on the stage. Carter wanted to hold their hands up in a sign of unity, instead he only could muster a cold handshake. The result was a Republican president, Ronald Reagan.

In his memoir, Carter, still bitter about what happened, wrote that the news stories about the incident "emphasized his lack of enthusiasm as an indication that the split in our ranks had not healed. This accurate impression was quite damaging to our campaign, and was to linger for a long time."

These kinds of tensions don't have to play out this way, but the candidates can't take unity for granted. Whoever wins and whoever loses will need to work extraordinarily hard to bridge these two camps.

The good news is that polls show at this point in the 2008 campaign there were a greater number of Clinton supporters who said they would never vote for President Obama. Today, 25% of Sanders supporters say they would not support Clinton if she won the nomination. In May 2008, about half of Clinton backers said they wouldn't back Obama.

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Yet the kind of anger that was evident in Brooklyn suggests the party is closer to a tipping point than it has been at any point in this campaign. If Democrats want to avoid a replay of 1968 or 1980, with a Donald Trump or Ted Cruz winning the White House, they are going to have to lower the temperature in the room and find ways to remind their supporters that the cause of party is more important than the imperative to be right.