Sen. Cory Booker publicly lashed out at his "friend" Joe Biden on Tuesday, but his real, unmentioned target was Kamala Harris, another rival for the 2020 Democratic nomination.

It was almost a month ago when Harris, the California senator, upstaged Booker and the rest of the crowded Democratic presidential primary field by confronting Biden for his opposition to court-ordered busing to desegregate public schools in the 1970s.

Harris' attack exposed Biden as unprepared and painted him as a tone-deaf, old-guard Democrat who once boasted of his ability to work with segregationist senators.

Biden seemed befuddled. Harris exhibited an instinct for the jugular and ready to get into the ring with Donald Trump.

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That attack gave Harris a bounce, both in polls and, more importantly, in fundraising. Although her second-quarter collections trailed behind those of Biden and other top contenders, Harris raked in $2 million in the 24 hours after the debate, nearly 17 percent of the entire haul for the second quarter.

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Meanwhile, Booker's tepid second-quarter fundraising of $4.5 million was slightly down from his first quarter. It further solidified Booker's standing as a second-tier candidate still waiting to break out of the bottom of the pack.

So Booker, the "lead with love candidate," this week pounced on Biden, not long after the release of his criminal justice platform. It may not have given Booker the same buzz as Harris' debate performance, but his stinging rebuke Tuesday earned him the role of Biden's chief critic of the news cycle.

"Joe Biden had more than 40 years to get this right. The proud architect of a failed system is not the right person to fix it,'' Booker said in a statement.

Booker was referring to the 1994 crime bill, considered the most comprehensive attack on crime in U.S. history, which Biden co-sponsored. It won bipartisan support and President Bill Clinton's signature. The sweeping measure included the federal assault weapons ban and funding for the hiring of 100,000 new police officers, but it has since been seen as the cause of mass incarceration of racial minorities.

Biden's new plan attempts to reverse many of that law's more controversial features. He called for ending the disparity that placed stricter sentencing terms on offenses involving crack versus powder cocaine as well as an end to the federal death penalty, which the 1994 bill authorized as a potential punishment for an increasing number of crimes.

Biden's plan also would seek to create a $20 billion grant program to encourage states to reduce incarceration by increasing spending on child abuse prevention, education and literacy, as long as states eliminate mandatory minimum sentencing for nonviolent crimes.

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He also would expand the Justice Department's role in rooting out institutional misconduct by police departments and prosecutors. In effect, Biden, who stood at the front of the get-tough-on-crime parade a generation ago, now wants to be considered the man best suited to author a new era of reform.

Booker, who has made criminal justice a top priority of his Senate work and presidential campaign, was not about to be bumped from the new parade of reform.

“The 1994 crime bill accelerated mass incarceration and inflicted immeasurable harm on Black, Brown, and low-income communities,'' Booker wrote. "While it’s encouraging to see Vice President Biden finally come around to supporting many of the ideas I and others have proposed, his plan falls short of the transformative change our broken criminal justice system needs."

Booker's critique gives him the chance to highlight his own work on criminal justice, which has become a signature issue. He co-sponsored the "First Step Act," signed by Trump in December, which was widely considered the first significant move toward reducing the number of people in federal custody, according to the Brennan Center for Justice.

And in March, Booker introduced a sequel, the "Next Act," which would slice in half mandatory minimums for nonviolent offenders, reinstate voting rights for felons and eliminate the discrepancy between crack and powder cocaine sentences — an idea that Biden endorsed Tuesday.

Booker has also pledged to offer clemency to more than 17,000 incarcerated people serving time for nonviolent drug-related offenses.

His criticism also has the potential of cutting into Biden's strong support among African-American voters, a crucial Democratic Party voting bloc.

Tuesday's remarks also continue Booker's efforts to show that Biden can't be trusted to continue the cause of civil rights — he was among the first to lambaste Biden's fond recollections of working with segregationists. Those criticisms also gave Booker a brief moment back in the spotlight.

And it also gave a preview of Booker's plan for the next televised debate, on July 31 in Detroit, where he will again share the stage with Biden and Harris. He has dropped hints over the past week that he intends to target Biden over the 1994 crime bill, which he says "supercharged" the federal prison system with mass incarceration.

Booker made it clear that he's not going to cede the stage to Harris.

He often mentions how Trump and his racist rhetoric have brought the nation to a "moral moment'' of reckoning.

But candidate Booker is looking for any kind of a moment to jump-start his campaign. Harris did it in the last debate. Now Booker is waiting for his chance.