“If you think the last four years has been chaotic, divisive, toxic, exhausting, imagine spending the better part of 2020 with Bernie Sanders versus Donald Trump,” said former South Bend, Ind., mayor Pete Buttigieg.

“Russia is helping you get elected,” former New York mayor Mike Bloomberg told Sanders, “so you will lose to [Trump].”

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Businessman Tom Steyer told Sanders, “The answer is not for the government to take over the private sector.”

Former vice president Joe Biden pointed out that Sanders voted against an assault-weapons ban and in 2012 said "we should primary Barack Obama.”

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Sen. Elizabeth Warren (Mass.) declared that she “would make a better president than Bernie.”

And Sen. Amy Klobuchar (Minn.) informed Sanders that his “math does not add up" and that “we should pay attention to where the voters of this country are, Bernie.”

Opinion Who could win the Democratic primary? Use the Post Opinions Simulator to pick a state and see what might happen in upcoming primaries and caucuses.

Sanders scoffed, smirked, grimaced and glowered. “Not true!” he interjected, and “categorically incorrect!” He shook his head and waved his hand dismissively. “I’m hearing my name mentioned a little bit tonight — I wonder why,” the front-runner said.

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He chose to parry by shouting counter-assaults — against Bloomberg, against Buttigieg, against Biden. The others joined in, and soon it was an all-out food fight, with rhetorical mashed potatoes landing everywhere. Warren hit Bloomberg, who hit Sanders. Biden hit Klobuchar, while Steyer hit Sanders and Bloomberg. Biden hit Steyer, and Warren pummeled Bloomberg.

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Within the first few minutes, the CBS News moderators lost control. Candidates shouted at each other, talked over the moderators and interrupted at will. “He spoke over time, and I’m going to talk!” bellowed Biden. The audience cheered and booed as if watching professional wrestling.

The melee brought a sickening sense of deja vu.

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After the South Carolina primary four years ago, on Feb. 22, 2016, I wrote:

“Are Republican voters really choosing as their standard-bearer a man who preaches such hatred and spews such vitriol?

“No, they aren’t — at least not yet. But they may get Trump anyway.

“The good news is that only 32.5 percent of South Carolina Republicans voted for Trump. The bad news: Trump may not need the support of a majority of Republican voters to secure the nomination.”

We now see a mirror image of this happening in the Democratic race. Sanders has only 29 percent support in polls, but the fragmented field prevents any one candidate from emerging as the alternative — much as the crowded field of Jeb Bush, Chris Christie, Marco Rubio, Ted Cruz and the rest split the anti-Trump vote in the Republican primaries.

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If this field remains splintered, the Democrats will soon find it’s Sanders or nobody: Either Sanders wins the nomination outright or comes close enough that his angry supporters torpedo the nominee, assuring Trump’s victory. If Tuesday night’s dynamic holds, Democrats are on their way to opposing Trump with a 78-year-old socialist who recently suffered a heart attack, who has had nice things to say about nasty regimes around the word, and who has a $60 trillion spending plan without the means to pay for it.

Sanders’s opponents sounded the alarm.

“Not only is this a way to get Donald Trump reelected,” said Buttigieg. “We got a House to worry about. We got a Senate to worry about.”

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Said Bloomberg: “If you keep on going, we will elect Bernie. Bernie will lose to Donald Trump. And Donald Trump and the House and the Senate and some of the statehouses will all go red.”

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Yet the candidates couldn’t rise above their squabbles.

They vied to produce the best anti-Sanders barb. “Can anybody in this room imagine moderate Republicans going over and voting for him?” asked Bloomberg. Buttigieg accused Sanders of having “nostalgia for the revolution politics of the ’60s.”

But just as often the unfocused Democrats aimed their fire every which way. Warren took every opportunity to hammer away at Bloomberg, even using a question about Chinese manufacturing to denounce Bloomberg for failing to release his tax returns. She also invoked an allegation, denied by Bloomberg, that he once had told a pregnant employee to “kill it.”

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The audience booed.

“If we spend the next four months tearing our party apart, we're going to watch Donald Trump spend the next four years tearing our country apart,” Klobuchar warned.

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She’s right. The winner of Tuesday night’s debate was Trump.

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