Detroit, Michigan (CNN) Under relentless attack midway through Wednesday's Democratic presidential debate , Joe Biden said: "Everybody's talking about how terrible I am on all these issues."

And the former vice president's next sentence -- "Barack Obama knew exactly who I was" -- is how he fought back.

The second night in Detroit featured the most bitter exchanges of the 2020 primary race yet, with former Obama administration colleagues unloading on each other over immigration, Sen. Cory Booker of New Jersey blasting Biden's record on criminal justice and New York Mayor Bill de Blasio facing criticism from both protesters and Democratic rivals for not yet having fired the police officer who was accused of fatally choking Eric Garner.

Sen. Kamala Harris of California -- elevated to the top tier of the race after a polling bounce that followed the first debate -- wore a much bigger target, too, with several candidates piling on her health care plan and Rep. Tulsi Gabbard of Hawaii launching a scathing attack on Harris' tenure as California's attorney general.

The candidates were more focused on attacking one another than they were on demonstrating how they'd take on President Donald Trump -- though Trump did come into play for pithy criticism, with Sen. Kirsten Gillibrand of New York saying her first act as president would be to "Clorox the Oval Office."

Here are five takeaways from Wednesday night's debate:

1. The Biden pile-on

Biden's record was assailed by nearly every Democrat who shared the stage with him.

Gillibrand asked him, repeatedly, about an op-ed he had once written opposing a child care tax credit, which featured a headline claiming that Congress would be subsidizing "deterioration of family."

Biden deflected the question by touting his role in the passage of the Violence Against Women Act and fights for equal pay -- points in his record he was eager to highlight. But when moderators moved on to Harris, she was ready with another attack.

"Why did it take so long, when you were running for president, to change your position on the Hyde Amendment?" Harris said, pointing to Biden's reversal this year of his previous support for a measure that barred federal dollars from paying for abortions. Earlier in the evening, Biden was caught on a hot mic asking Harris to "go easy on me, kid."

Washington state Gov. Jay Inslee attacked Biden's climate change plan. Several candidates hit the former vice president on immigration. And one of the most memorable exchanges came when Booker assailed Biden over the passage of the 1994 crime bill, which critics point out has contributed to mass incarceration.

After Biden's lackluster performance in the first debate, his campaign had promised the former vice president would be much more prepared to brawl Wednesday night. And without a doubt, he was. He even landed some sharp blows himself, particularly against Harris on health care and her record as California attorney general.

The question, though, is whether Biden was damaged by the number of attacks he faced. Even though he fended off many of them, their sheer volume could plant doubts about one of his strongest assets -- his perceived electability -- in the minds of Democratic primary voters.

2. Piercing Biden's Obama shield on immigration

Biden -- who has placed his time as Obama's vice president at the center of his campaign -- struggled to answer Democratic foes who criticized Obama-era deportations of undocumented immigrants.

Biden was attacked by Booker, de Blasio and former Housing and Urban Development Secretary Julián Castro -- who, like Biden, served in the Obama administration.

"It looks like one of us has learned the lessons of the past and one of us hasn't," Castro said to Biden. "We need someone who actually has guts on this issue."

De Blasio pressed Biden on whether he had counseled Obama to halt the deportations.

"Did you say those deportations were a good idea or did you go to the President and say, 'This is a mistake. We shouldn't do it'?" de Blasio asked.

Biden said he wouldn't talk publicly about the advice he had given Obama privately on the issue -- and that's when Booker piled on.

"You can't have it both ways," Booker said. "You invoke President Obama more than anyone in this campaign. You can't do it when it's convenient and then dodge it when it's not."

3. Harris wears a bigger target

Harris' criticism of Biden's early-career opposition to federally mandated desegration busing rocketed her up in the polls after the first debate in June. And on Wednesday night, her Democratic rivals treated her as one of the front-runners.

Gabbard launched perhaps the most scathing attack of the night, knocking Harris' record as attorney general of California.

"There are too many examples to cite, but she put over 1,500 people in jail for marijuana violations and then laughed about it when she was asked if she ever smoked marijuana," Gabbard said.

Harris' response was similar to how Biden had defended himself on a range of issues: She argued that her record reflects she has a long history of working to overhaul the criminal justice system.

Biden, who entered the race decrying the "circular firing squad," showed he had been picking through Harris' record, too.

He faulted Harris for defending cases that were found to have involved prosecutorial misconduct and for not disclosing more quickly to defense attorneys that evidence in the San Francisco crime lab had been tampered with.

"Google '1,000 prisoners free Kamala Harris,' " he told viewers.

Campaigns sometimes talk about the perils of peaking too early. While most would rather be in Harris' position right now, the criticism she faced was a reminder that emerging as a top-tier contender comes with increased scrutiny.

4. Biden and Harris clash on health care

The debate opened with a fierce battle over health care that revealed how both Biden and Harris are approaching policy matters.

Biden defended his Obamacare 2.0-style plan, which would put in place a "public option" that would allow people to buy into a government-run program but also keep private insurance.

"No one has to keep their private insurance," he said. "They can buy into this plan and they can buy into it with a $1,000 deductible and never have to pay more than 8.5% of their income when they do it, and if they don't have any money they will get in free."

But Harris pointed out that Biden's plan would leave 10 million Americans without coverage.

"In 2019 in America, for a Democrat to be running for president with a plan that does not cover everyone, I think, is without excuse," she said.

Harris touted what she is calling a "Medicare for All" proposal that would, over 10 years, enroll all Americans in a program paid for by the federal government. Her plan would use private insurers as the vehicle to deliver that coverage and she says she would not increase taxes on middle-class Americans, both important breaks from Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts. Under their proposals, the private insurance industry would be eliminated.

Biden, though, tore into the argument by supporters of Medicare for All that the program would come without a deductible that Americans would have to pay.

"This idea is a bunch of malarkey that we're talking about here. The fact of the matter is that there will be a deductible -- there'll be a deductible in the paycheck," he said, referring to taxes he argued would be necessary to pay for a fundamental overhaul of the American health care system. He credited Sanders with acknowledging as much, but accused Harris and de Blasio, who also argued for Medicare for All, of fuzzy math.

He twisted the knife when criticizing the lengthy phase-in of Harris' plan, saying: "Anytime someone tells you you're going to get something good in 10 years, you should wonder why it's going to take 10 years."

5. Booker breaks through

Booker has languished at the bottom of the polls and needed to stand out. On Wednesday night, he did, delivering a commanding performance that placed him at the center of the action without turning himself into a target.

Early on, he stayed out of the fray on health care, arguing that Trump benefits from pitting progressive Democrats against moderates.

Later, he got the best of Biden during exchanges on immigration, criminal justice and climate change.

When Biden tried to defend himself by pinning blame on Booker for racist practices of the Newark, New Jersey, police department, Booker shot back: "There's a saying in my community. You're dipping into the Kool-Aid and you don't even know the flavor."

Later, when the candidates were discussing climate change, Booker retorted: "Nobody should get applause for rejoining the Paris climate accord. That is kindergarten."

He also won over the Detroit crowd by warning Democrats to be ready for an "all-out assault" on African Americans' ability to vote in the 2020 election.

"We lost the state of Michigan because everybody from Republicans to Russians were targeting the suppression of African American voters," he said. "We need to say that."