(CNN) Former Vice President Joe Biden on Wednesday previewed sharp new lines of attack on two of his Democratic 2020 rivals, New Jersey Sen. Cory Booker and California Sen. Kamala Harris , one week ahead of an anticipated debate-stage showdown.

Hours after Booker had called Biden an "architect of mass incarceration," Biden fired back by pointing to Newark police practices during Booker's time as mayor.

"His police department was stopping and frisking people, mostly African American men," Biden said, adding that Booker "objected to federal interference" at the time.

Biden also criticized Harris, without naming her, for backing a Medicare-for-All proposal while claiming she would enact it without raising taxes on middle-class Americans.

"Come on. I mean, what is this? Is this a fantasy world here?" Biden told reporters after a campaign stop in Dearborn, Michigan.

Biden's pointed comments reflect how the former vice president, who entered the Democratic nominating contest focused squarely on demonstrating to the party's voters that he would be the best candidate to take on President Donald Trump, is adapting to the realities of being a front-runner under siege in a competitive primary.

Harris and Booker will both stand next to Biden in Detroit next week, when all three are on stage for the second night of the Democratic presidential debate hosted by CNN

Harris climbed in the polls after launching a withering attack during the first debate on Biden over his previous opposition to federally-mandated busing. Booker, meanwhile, has in recent days signaled that he intends to attack Biden's record on criminal justice.

At a fundraiser later Wednesday night, when an attendee pressed Biden to be tougher in the upcoming debate, Biden was even more direct, saying, "I'm not going to be as polite this time."

In a shot at Harris, he said it wasn't long ago that she'd been eager to ingratiate herself with him.

"This is the same person who asked me to come to California and nominate her in her convention," Biden said.

He also signaled he is willing to attack elements of Booker's and Harris' records -- a reversal for a candidate who, just weeks ago, said of the reams of opposition research dug up on his opponents: "I'm just not going to go there."

"If they want to argue about the past, I can do that," Biden said Wednesday night. "I got a past I'm proud of. They got a past that's not quite so good."

Biden , then a Delaware senator, helped write the 1994 crime bill, which -- in addition to putting 100,000 new police officers on the streets and creating an assault weapons ban and the Violence Against Women Act -- also expanded the federal death penalty and created dramatically harsher sentencing laws. This included the three strikes law, mandatory life terms for people with at least three federal violent crime or drug convictions. One of the most controversial aspects of the crime bill was granting states billions to build prisons if they passed their own tough sentencing laws with mandatory minimums.

This week, Biden unveiled his criminal justice reform plan , which goes in the opposite direction of laws he helped enact two decades earlier. His proposal includes a $20 billion grant program aimed at reducing incarceration and pledges to use his clemency power as president to reduce or pardon prison sentences for non-violent drug offenses.

With much of the Democratic field in Detroit for an NAACP gathering Wednesday, Booker argued that Biden's proposal is inadequate.

"For a guy who helped to be an architect of mass incarceration," Booker told reporters in Detroit, "this in an inadequate solution to what is a raging crisis in our country -- that we have 5% of the globe's population but 25% of the globe's prison population, we have the overincarceration of low-income folks, veterans folks, addicted folks, mentally ill folks and disproportionally black and brown folks."

Biden, asked about Booker's remarks later Wednesday, told reporters, "Cory knows that's not true."

He also pointed to the crime bill's provision allowing the Justice Department to conduct "pattern or practice" investigations into police misbehavior -- and noted that Booker "objected to federal interference" when the department, under former President Barack Obama, launched a probe in Newark.

"If he wants to go back and talk about records, I am happy to do that -- but I'd rather talk about the future," Biden said.

The remarks showed that Biden has been studying his opponents' potential weaknesses and -- unlike his approach to the first debate -- he is prepared to fight back next week.

Biden added that he is "anxious to have a debate" with Booker and said, "I know Newark pretty well."

Shortly after Biden's criticism of Booker, the former vice president's campaign released a statement from deputy campaign manager Kate Bedingfield calling the Newark police department "a civil rights nightmare" during Booker's tenure as mayor.

"To his credit, more recently, Booker has been a leader in criminal justice reform. For decades, Joe Biden has been working on criminal justice reform," Bedingfield said -- in a remark that also underscored Biden's desire to focus on more recent events, including his time as Obama's vice president, and his campaign's proposals.

Booker's campaign manager, Addisu Demissie, responded on Twitter, highlighting the portion of Bedingfield's statement saying Biden had been working on criminal justice reform for decades.

"That's the problem," he said.

"For decades, Joe Biden has been working on criminal justice reform."



That's the problem. And that's the tweet. pic.twitter.com/tOFpRI4icG — Addisu Demissie (@ASDem) July 24, 2019

Booker had been pressed Sunday in an interview with CNN about a Justice Department report that found police practices hurt black residents of Newark during his tenure as mayor.

"I actually took responsibility," Booker said. "Most folks who know New Jersey know I inherited a police department that had decades of challenges with accountability, challenges along racial lines."

"And we actually stepped up to deal with the problem, not only working with the DOJ, but working with the ACLU to put forward what was a national standard-setting level of accountability," he said.

Booker said he is "proud of that record as a mayor of trying to take on these problems. And, frankly, as a senator, I have been one of the few senators that have put forth legislation to create more police accountability, more transparency, so that we could begin to deal with some of the deep racial issues that we have that permeate not just policing, but also everything from prosecutorial actions to our criminal justice system as a whole."

Biden also appeared to be sharpening his attack on Harris -- without using her name.

He said that Vermont Sen. Bernie Sanders has "been honest" about the costs and ramifications of Medicare for All -- including saying that it would involve higher taxes for all Americans, though Sanders' campaign notes that people would pay no health insurance premiums or deductibles and would save money overall.

Harris, though, has said she would enact Medicare for All without a middle-class tax increase.

The former vice president said, "I think we should have an honest debate. Look, you can't go out and you can't run for president and beat Donald Trump without leveling with the American people about what you're going to do and how you're going to pay for it. I'm anxious for that debate."

Biden also again previewed a new approach in the upcoming debate, saying he "was probably overly polite in the way I did respond to an attack" in the first debate when Harris lambasted his record on busing.

Asked if he's going to be less polite at the next debate, Biden said, "I'm going to smile a lot."