MIAMI, Florida—There is blood in the water and the campaigns know it.

For weeks, Democrats involved in the primary have privately warned that Joe Biden was a paper tiger—a frontrunner, for sure, but one with clear vulnerabilities and a defensive campaign approach. On Thursday night, in front of the largest primary audience thus far, they found their vindication.

The second night of a back-to-back debate in South Florida, the first of a dozen, featured four of the five top-polling candidates in the sprawling 2020 field. But it was a five-minute exchange between Sen. Kamala Harris (D-CA), the only African-American candidate onstage, and Biden that changed the tenor of the two-hour broadcast and perhaps the course of the primary itself.

Harris’ blistering and personal critique of Biden’s position on school-desegregation busing in the 1970s placed the former Veep on the defensive. It also put the biggest dent yet into his greatest political asset: the aura of electability and inevitability that he has tried to build during his time on the trail. In the hours that followed, the other campaigns in the race rushed in to make the point that the man many perceive to be best equipped to defeat President Donald Trump was, in fact, not so formidable.

“I thought Biden’s response was surprising in the sense that I thought we as a party had moved beyond debating whether the federal government has an active role in addressing school desegregation in the United States, but apparently we’re still having a little debate and contrast over that,” Faiz Shakir, the campaign manager for Sen. Bernie Sanders (I-VT), told The Daily Beast. “I think coming out of last night’s debate, I personally am far more concerned about his ability to go up against Trump in a debate. I certainly lost some confidence as a Democratic voter who wants to defeat Donald Trump if he’s our nominee.”

Even those more ideologically predisposed to rush to Biden’s defense acknowledged on Friday that the evening had been a let down.

“It wasn’t good,” said Matt Bennett, Vice President for Public Affairs and a co-founder of centrist-Democratic think tank Third Way. “He seemed unprepared—it reminded me of Obama’s first debate with Romney. And my guess is that it was the same dynamic—a very experienced guy semi-ignores the prep thinking he is ready, and he’s not. He can certainly recover from this, and I expect that he will, but it’s going to take some work.”

Biden has overcome hiccups already on the trail. But Thursday night’s debate showing seemed to be of a different—more consequential—variety. The former VP didn’t stick around to spin his performance after the debate was over. But his surrogates rushed into that void. One of his national campaign co-chairs, Rep. Cedric Richmond (D-LA), said that formats like Thursday’s were not best suited to discuss difficult issues.

“It’s a debate and in a debate, you have 30 seconds to respond give or take,” he said. “It’s not the climate to go into very nuanced arguments and debates.”

Talking points distributed by the campaign and obtained by The Daily Beast encouraged surrogates to stress that Biden had been “very clear” about his reasons for running for president and “spoke directly to the American people” about his “bold vision.”

Statistically, the damage that Biden’s candidacy endured on Thursday night is hard to quantify, in large part because the data is not complete. But preliminary surveys show a complicated picture. Data compiled by the Democratic polling firm Democracy Corps showed Biden’s favorability with African-American voters actually going up a net 18 percent after the debate. Stan Greenberg, the longtime party pollster, chalked much of that improvement up to defensiveness over the perception that the attacks on Biden were attacks on the Obama-Biden legacy. But the same survey also showed that the percentage of people who would vote for Biden and consider voting for him went down 11 points from 81 percent to 72 percent.

“I think the pain that Kamala Harris raised, the kind of personal pain, I was surprised that the vice president did not actually respond to that and got defensive,” said Randi Weingarten, the president of the American Federation of Teachers, which sponsored the Democracy Corps survey. “I know him as one who in a non-debate setting would have been actually much more responsive. It may have been just the debate setting or things like that.”

Elsewhere the polling picture looked bleaker.

By Friday, the former vice president was notably more energetic in making his case. His campaign sent out an email to supporters, titled, “I'd like to say something about the debate last night,” in which Biden said—in the second line—“I heard and I respect Senator Harris.” His campaign team branched out to the various cable networks to stress his record of pursuing civil rights legislation. And at a labor luncheon held by the Rainbow PUSH Coalition, Biden insisted that he “never, never, never ever opposed voluntary busing,” despite the fact that he had been publicly critical and opposed to federally-mandated busing during his Senate career.

“Folks,” Biden stressed, “the discussion in this race shouldn’t be about the past.”

But even if the discussion does turn to the future, the dynamics of the race seem less favorable for Biden than they appeared just 24 hours ago. For other campaigns, Thursday night’s debate provided what amounted to a green light to lay their own body blows on the former VP. Several gave it a go during the debate itself with Rep. Eric Swalwell (D-CA) imploring Biden to “pass the torch” to a younger generation and Sen. Michael Bennet (D-CO) lacing into him for negotiating a deal that permanently extended a massive chunk of the Bush tax cuts with Senator Mitch McConnell (R-KY). And others continued it after the debate had ended.

“This is a contact sport,” former Colorado Governor John Hickenlooper said. “The bottom line is: He’s a big boy. It’s not his first rodeo.”

As Sen. Sanders made his way through a throng of reporters following the debate’s conclusion, he declined to answer a question about whether Biden owed an apology for his prior comments about an era of civility in the Senate in which he was able to work with segregationist senators despite their disagreements, saying he had already addressed that.

Asked about what it was like to be wedged between Harris and Biden and whether he was surprised at the intensity of the exchange, the senator flashed a toothy grin.

“No,” he told The Daily Beast.

—Hanna Trudo contributed reporting