Key moments from Wednesday's Democratic presidential debate

Former Vice President Joe Biden was at center stage and on his rivals' minds at Wednesday's Democratic presidential debate. Biden, despite what many viewed as a wobbly performance in last month's debate, remains the front-runner in polls. That made him a target for Sen. Kamala Harris, with whom he tangled at the June debate, and for most of the other candidates, who went after him on immigration, health care and criminal justice in hopes of distinguishing themselves and denting his lead. Stricter qualifying criteria for the Sept. 12 debate mean some candidates won't be included. Here's a look at some of Wednesday's flash points. Read highlights from Tuesday's debate with 10 other Democratic candidates.

Impatient with other candidates' feuding, Bennet objected to "debating what people did 50 years ago. … We need a conversation about what's happening now." When some children don't get pre-K education and Detroit public schools remain segregated, he said, "equal is not equal." He said 88% of people in prison dropped out of high school. "Let's fix our school system and maybe we can fix the prison pipeline."

At the debate's outset, he noted that "Democrats are expecting some engagement here and I expect we'll get it." In the debate's first hour, he repeatedly challenged Harris over her health care plan and told her, "You can't beat Donald Trump with double talk." He also defended the Affordable Care Act enacted during his White House tenure and referred to calls by Harris and others for Medicare for All as "a bunch of malarkey."

Booker's opening comments were interrupted by hecklers demanding the firing of the New York City officer who put Eric Garner in a fatal chokehold in 2014. The demand was apparently directed at de Blasio. Later, after several candidates sparred over health care, Booker warned that Trump was probably enjoying the dispute. He added that the right approach is to "end this broken system."

The former housing secretary called open borders "a right-wing talking point" and said the only way to prevent the separations of children from their parents is to repeal the law the Trump administration is using to do so. He has proposed decriminalizing illegal border crossings and putting immigrants on a path to citizenship. Such an approach, he said, would be "smarter, more effective and more humane."

In his opening statement, the New York mayor went after Biden for saying he doesn't stand for dramatic change and Harris for not wanting to restructure society. "We can make change in this country. I know from personal experience it can be done," he said, citing his city's minimum-wage increase and decrease in crime. "When I am president, we will even up the score and we will tax the hell out of the wealthy," he said.

The congresswoman from Hawaii blasted Harris after an exchange about criminal justice. Harris, a former prosecutor and state attorney general "is proud of her record," Gabbard said, "but I'm deeply concerned." Harris, she said, jailed 1,500 people for marijuana offenses then laughed when asked if she ever smoked marijuana and "kept people in prison beyond their sentences to use them as cheap labor."

Gillibrand called it her responsibility "as a white woman of privilege" to join Harris and Booker to focus on race. She can explain to white suburban women who supported Trump that when their sons walk in their neighborhoods wearing hoodies or seek help after their cars break down, "his whiteness is what protects him from getting shot." "Together we can make our communities stronger," she said.

"Go easy on me, kid," Biden joked to the California senator during the candidates' introductions. She didn't. She defended her health care plan, which would preserve private insurance, when asked about Biden's description of it as a "have-it-every-which-way approach." He said her plan would cost $3 trillion. Her rebuttal: "Unfortunately, Vice President Biden, you're just simply inaccurate in what you're describing."

The Washington governor joined the chorus of Biden critics, calling his plans to fight climate change inadequate. Inslee said the challenge encompasses "all the issues that we Democrats care about," including health care, national security and the economy. "Middle-ground solutions like the vice president has proposed or sort of middling, average-sized things are not going to save us," he said.

Yang intervened after several candidates criticized Biden for his role in crafting the 1994 crime bill that imposed stricter sentences for crack-cocaine possession. "I would trust anyone on this stage much more than I would trust our current president on criminal justice," he said. Biden has unveiled a plan to reform the criminal justice system and end racial and income disparities in sentencing.