DETROIT — Former vice president Joe Biden took aim at his critics in the presidential field Wednesday, scoring Sen. Cory Booker’s record and Sen. Kamala D. Harris’s policy positions in a bracing new offensive that signaled a more aggressive stance heading into next week’s presidential debate.

Biden’s response came as the candidates gathered here for the NAACP’s 110th convention, a day after the release of his new criminal justice proposal that served as a stark contrast to the 1994 crime bill he championed. Biden has drawn repeated criticism for that bill, which opponents argue increased imprisonment particularly among black men. Booker tweeted a tart critique Tuesday.

“It’s not enough to tell us what you’re going to do for our communities, show us what you’ve done for the last 40 years. You created this system. We’ll dismantle it,” Booker tweeted.

On Wednesday, asked by NAACP panel moderator April Ryan if his views have evolved, Biden said the focus of criminal justice needed to shift from “incarceration to rehabilitation” — a solution that Booker, speaking to reporters after his earlier NAACP appearance, called “inadequate.” He described Biden as “an architect of mass incarceration.”

After Biden left the stage, he responded emphatically to Booker’s second day of criticism by trying to turn attention to his tenure as mayor of the troubled city of Newark, before he became a senator.

“Cory knows that’s not true, number one. Number two, you know the significant portion of the incarceration that occurred before the crime bill was written,” Biden told reporters before taking a shot at police practices during Booker’s tenure.

“His police department was stopping and frisking people, mostly African American men,” he said, leading the Obama administration to intercede. “We took action against them; the Justice Department took action against them, held the police department accountable.

“He objected to federal interference. If he wants to go back and talk about records, I’m happy to do that. But I’d rather talk about the future . . . I challenge him or anyone else to tell me how he has a better plan than I have for moving from here.”

Later, Biden’s communications director, Kate Bedingfield, sent out a statement that set up another showdown at next week’s primary debate, to be held in Detroit.

“Since next week’s debate format will give Senator Booker twice as much time to make his attacks than it allows Vice President Biden to respond to them, we thought we would begin to respond now,” Bedingfield wrote. She then outlined Booker’s support for Newark’s police enforcement of a “zero tolerance policy” for minor infractions in 2006, as well as the department’s reliance on “stop and frisk” treatment of residents, a practice that has long been condemned by Democrats and civil rights activists.

The dust-up between the two was the second of the campaign; in the approach to June’s primary debate, Biden and Booker tussled over Biden’s praise for his past habit of working with segregationist senators. After Booker called on Biden to apologize, Biden defiantly refused and insisted Booker should be the one to apologize.

The conflict between the two men did not cascade into the debate, however, because Booker was placed in the first of two stages, and Biden was on the second. During that second session, it was Harris who went after Biden, challenging his past opposition to federally mandated busing and contrasting his record with her experience of being bused as a child.

Biden and Harris will again share a single stage in next week’s second debate — this time alongside Booker — and in preparation the former vice president targeted her as well on Wednesday. He credited himself for her post-debate rise in multiple national polls because, he told reporters, he was “overly polite” in responding to her criticism.

“I didn’t respond to an attack: ‘You’re not a racist.’ Which is a nice thing to say. It was really reassuring,” Biden said in a sarcastic tone.

Without using her name, Biden also snuck in a dig at Harris’s support of Medicare-for-all, which she says would eliminate the need for private insurance beyond supplemental coverage. Asked about his approach to the next debate, Biden brought up health care as a subject on which he will be ready to spar.

“I have some people saying, ‘No, I’m not going to do away with Obamacare. We can still have supplemental insurance,’ ” Biden said. He contrasted Harris’s insistence that she would pay for the enormously expensive program with a tax hike on the wealthy and Wall Street, whereas the measure’s author, Sen. Bernie Sanders (I-Vt.) has said he will raise taxes on the middle class.

“Bernie’s been honest; he’s going to raise taxes on middle class,” said Biden, whose health-care plan would expand Obamacare and create a government-run insurance plan to compete with private insurers.

“Well, you’ve got to find 30 to 40 trillion dollars somewhere. And how are you going to do it? Well, I find that people would say that, for Medicare-for-all, they’re not going to tax the middle class because you don’t need to do that!” Biden added. “Come on! What is this, is this a fantasy world here?”

Sanders’s campaign issued a sharp reply, thanking Biden for his praise but accusing him of continuing to lie about the Medicare-for-all plan.

The back-and-forth between candidates came as several of them arrived at the NAACP meeting to make the same argument: that President Trump is racist and wants to undermine progress made by the civil rights movement.

William Weld, the former Massachusetts governor who has launched a long-shot Republican primary challenge to Trump, said the president represented an existential crisis for their party.

“Unless the national Republican Party in Washington expressly, expressly rejects the racism of Donald Trump,” Weld said, “it will become universally viewed as the party of racism in America.”

As candidates touted their plans for minority communities, Biden used his role as President Barack Obama’s running mate as an endorsement of his character.

“They did a significant background check on me for months with 10 people,” Biden said. “I doubt he would have picked me if those accusations about my being wrong on civil rights was correct.”