The gloves will be off during this week's Democratic debates. Here's what to watch for

Here are the Democrats who will confront each other during this week's debates, and try to create a moment that will carry their campaign to the next debate.

WASHINGTON – Joe Biden said he "wasn't prepared."

The former vice president frowned on the debate stage last month as Sen. Kamala Harris criticized his "hurtful" comments on working with segregationist Democratic senators and his opposition to federally mandated busing to integrate public schools while he was in Congress.

Harris described a little girl who was helped by busing from her school district in the Bay Area, closing with the line: "That little girl was me."

Though Biden aggressively pushed back against the criticism, pundits and many viewers alike said Harris came out on top in the exchange. The confrontation made headlines and led to a bump in polls for Harris over the next several weeks.

Since then, Biden has promised: He's done being polite.

He's not alone. Several candidates escalated their rhetoric against fellow 2020 Democrats before the next round of debates Tuesday and Wednesday at the Fox Theatre in Detroit.

The gloves are officially off. Here is what to watch for:

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What's at stake?

This debate is crucial for many Democrats – especially lesser-known candidates (such as Govs. Steve Bullock or Jay Inslee) or candidates who have seen steadily dipping poll numbers (Beto O'Rourke) if they want to make it onto the next debate stage, or even keep their campaigns running until the Democratic caucuses and primaries begin in February.

The threshold for the debates in September requires candidates to hit 2% in four qualifying polls and tally at least 130,000 individual donors, according to the Democratic National Committee guidelines. Seven candidates – O'Rourke, Biden, Harris, South Bend, Indiana, Mayor Pete Buttigieg and Sens. Cory Booker, Bernie Sanders and Elizabeth Warren – have met the threshold.

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Biden vs. Harris and Booker

On the second night of the debates, Biden will be sandwiched between the two African American presidential candidates, both of whom have been critical of his record on civil rights. Harris and Booker are vying for the critical black voter bloc – one Biden continues to hold on to, according to polls.

Throughout the debate, the three candidates will probably try to tout their records as best on criminal justice and civil rights. The debate will take place in Detroit where nearly 80% of the city is African American, according to the U.S. Census Bureau.

Last week, Biden introduced a criminal justice plan that would reverse portions of a 1994 crime bill that he helped write. Harris introduced a plan in the Senate that would decriminalize marijuana, a change from her previous stance.

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Booker slammed Biden as the "architect" of mass incarceration after Biden introduced his criminal justice plan. Booker pointed to the 1994 crime bill, which critics said targeted and affected mostly communities of color.

"Cory knows that's not true," Biden shot back, telling people to look at Booker's record while mayor of Newark, New Jersey.

"(Booker's) police department was stopping and frisking mostly African American men," Biden said. "If he wants to go back and talk about records, I'm happy to do that. But I'd rather talk about the future."

Booker, who has lagged in polling, said Friday that Biden's criticisms were "ridiculous."

"I will always speak truth to power," the New Jersey Democrat told MSNBC. "The response to having a substantive conversation about people's records shouldn't be to go onto the attack."

Biden signaled that he won't be afraid to fight back if Harris comes after him again.

“I’m not going to be as polite this time, because this is the same person who asked me to come to California and nominate her in her convention,” he said while fundraising in Detroit last week. Biden spoke in 2016 on behalf of Harris for her Senate campaign at the California Democratic convention.

Warren and Sanders vs. the moderates

Two of the most liberal 2020 candidates will be on stage with more moderate candidates.

Warren and Sanders have argued for a $15 federal minimum wage, free college and tax increases on the wealthy. The two New England senators have called for canceling student loan debt.

Candidates such as former Rep. John Delaney, former Colorado Gov. John Hickenlooper, Sen. Amy Klobuchar, Rep. Tim Ryan of Ohio and Montana Gov. Bullock have touted their campaigns as alternatives to the far-left presidential hopefuls.

Last week, Bullock criticized plans that would cancel student loan debt. He said one-third of the debt is owed by the wealthiest quarter of all Americans and only 20% of Americans carry student debt at all.

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"They don't need more big promises that look good in a tweet but fail to deliver real change," Bullock wrote in an op-ed for CNN. Bullock didn't qualify for the first debate and will introduce himself to a wide national audience for the first time.

Hickenlooper has repeatedly denounced "socialism" (Sanders is a self-described democratic socialist), and Klobuchar touted her bipartisan work in the Senate.

Warren and Sanders routinely poll among the top candidates with voters. None of the more centrist Democrats sharing the stage with them has broken into the top five.

The candidates who need a moment

Several candidates will look for a breakout moment.

O'Rourke, once seen as a top contender, has slumped in polling and fallen behind in fundraising.

Buttigieg, who raised the most money among 2020 Democrats last quarter, has seen his poll numbers plateau. He has struggled to gain traction with black and Latino voters, groups he needs to have a chance to win the election.

Julián Castro, who was Housing and Urban Development secretary in the Obama administration and is the only Latino candidate, has hardly broken past 1% in many polls.

Sen. Kirsten Gillibrand, one of six women running for president, hasn't made much headway in a crowded field. Despite routinely interjecting her policy ideas in the last debate, the New York Democrat has not seen support grow.

O'Rourke and Castro have at least one poll above 2% and have more than 130,000 donors. Gillibrand has yet to make 130,000 donors or poll at 2%.

O'Rourke is one of the only candidates who talked about how he watched his debate performance from last month. During an interview with Jemele Hill, he said he had "tunnel vision" and was "focused on the question and your response and just what you feel" and not everything that was happening on stage.

He said he is more comfortable in a town hall setting or talking to voters one on one, but "the onus is on me to do a better job of conveying" his experience on stage and through the TV screen.

"How I get that across in that one-minute answer on the debate stage, how I convey that in a national television interview, that's on me," he said.

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This article originally appeared on USA TODAY: Democratic debate: Biden versus Booker and Harris, and other matchups