Twenty candidates are sharpening memorable lines and crafting strategies for butting into the conversation as they prepare for a second round of Democratic presidential debates that are expected to be explosive, particularly over topics of race, inequality and criminal justice.

Former vice president Joe Biden, having concluded he was not aggressive enough in the first clash in June, has practiced criticizing his rivals on health care and other issues as he prepares for Round 2, which begins Tuesday night at 8 p.m.

Sen. Bernie Sanders (I-Vt.) is again being advised to draw a sharp contrast with Biden, even though they will not be sharing the same stage. Before last month’s debate, there was a push inside the campaign for Sanders to hit Biden hard, according to people familiar with the talks. One idea was to point to comments in which Biden had assured wealthy donors that “nothing would fundamentally change” for them if he won the presidency.

Ultimately, Sanders never fully launched the attack, raising questions about what he will do this week.

Biden, who was pummeled in the last debate by Sen. Kamala D. Harris (D-Calif.) and others, has shifted most dramatically since then. The candidate, who three weeks ago said he saw no reason to dig into the past records of his opponents, has been doing just that. The candidate, who once warned against a “circular firing squad,” is now joining the circle.

Bakari Sellers, a former South Carolina lawmaker who supports Harris, was among those arguing that Biden is the candidate under the most pressure to perform well.

“He can’t display a glass jaw in back-to-back debates,” Sellers said. “If he does that, his support will creep away.”

Onstage Wednesday night, Biden will be standing between Harris and Sen. Cory Booker (D-N.J.), who have been critical of Biden’s record and are prepared for volatile and personal exchanges. On Tuesday, the center stage will be held by the liberal pairing of Sanders, who came in second in the 2016 Democratic contest but has lost ground of late, and Sen. Elizabeth Warren (D-Mass.), who has overtaken him in some polls.

The two-day period represents the next crossroads in the Democratic contest, and for some candidates, it could also be their last stand. More than half of the field is at risk of falling short of the polling and donor thresholds needed to appear in the third debate in September, making this week perhaps their final opportunity to shake things up.

That has given even more emphasis to the lesson learned in June: On such a crowded stage, it pays to break the rules. Interrupt if you have to, talk longer than the allotted minute if you must, and, above all, seek a controversy-stirring moment that cable television will play for days afterward.

That was the preplanned recipe Harris followed in the last debate. The result of her June joust with Biden was that she, more than any of the 20 candidates onstage, leaped in the rankings. She also got a fundraising boost from the sale of a T-shirt that played off her key moment, when she contrasted Biden’s opposition to busing for integration with her experience as a child bused to school.

Some candidates are preparing less on how to deliver their own lines than on how best to interrupt their opponents.

“One of the things that we learned is that it is a total free-for-all. It was like a children’s soccer game, 10 candidates swarming the ball,” said a senior adviser to one of the candidates, who, like many interviewed, spoke on the condition of anonymity to discuss internal deliberations. “We have done a lot less practice of what your 60-second answer is and a lot more practice of live-fire drills of how to interject into the debate.”

Biden’s new posture has been evident since the day after the last debate, when he defended his civil rights record more definitively than he had onstage the night before. Biden would generally brush aside opportunities to criticize any of his opponents, focusing almost exclusively on President Trump.

“I get all this information about other people’s pasts, and what they’ve done and not done — and, you know, I’m just not going to go there,” Biden told CNN. “If we keep doing that — I mean, we should be debating what we do from here.”

He has since sharpened his attacks on Sanders for his signature health-care plan, which he has said would raise taxes on the middle class even as it lowered health-care costs. He has also recently called the notion that young voters want revolutionary change “fiction” and declared, “They aren’t a generation of socialists” — a seeming reference to Sanders’s brand of democratic socialism.

Over the past week, with his record under scrutiny, Biden began criticizing Booker’s backing of the Newark Police Department’s “stop and frisk” policy while he was mayor — with such ferocity that Booker’s campaign was taken aback. Biden also took issue with Harris after she said she did not support a middle-class tax hike to pay for the Medicare-for-all plan she endorsed; only in “a fantasy world” would the tax hike not be necessary, he said.

Biden said he felt like a target going into the second debate: “As long as you’re leading, you’re the target.” He added, “I’m looking forward to it.”

Biden’s campaign has been bracing for attacks from multiple candidates beyond Harris and Booker, with critiques on his record on trade and workers’ rights from New York Mayor Bill de ­Blasio, on women’s and abortion rights from Sen. Kirsten Gillibrand (D-N.Y.), and on immigration policy from former San Antonio mayor and housing and urban development secretary Julián ­Castro.

“Everyone is looking for their T-shirt moment,” one Biden campaign official said. “And Joe Biden thinks this is bigger than selling T-shirts.”

Biden’s campaign advisers think they could benefit most from a debate over health care. He has argued strenuously for expanding the Affordable Care Act rather than establishing a fully ­government-run system.

The two biggest proponents of Medicare-for-all — Sanders and Warren — will not be on the stage with him, but Booker, Harris and Gillibrand, who are co-sponsors of the legislation, will be. Harris, in particular, has stumbled over how far she would go in ending the private insurance ­system.

Harris has largely ignored her opponents lately, although she has sought to distinguish herself from them in key ways. Some Harris allies don’t expect her to be looking for a fight.

“I kind of expect Senator Harris to take a more presidential approach to the debate and only smack when being attacked — which may be frequently,” Sellers said.

For the first debate night, however, much of the pressure will be on Sanders to regain his footing after a June performance that was overshadowed by Harris and Biden. Sanders and his team have been preparing for Round 2 more rigorously, according to the people familiar with the campaign talks.

Although Sanders will be onstage with Warren, campaign officials are advising him to focus on other targets, most notably Trump and Biden, according to these people. Sanders has appeared eager to participate in the dispute with Biden over health care. Advisers are also encouraging Sanders to emphasize his core economic message.

“The senator needs to make the case that he has the best economic argument against Donald Trump, especially in the three states that we need to win the presidency: Wisconsin, Pennsylvania and Michigan,” said Rep. Ro Khanna (D-Calif.), a national co-chair of the Sanders campaign.

However, one big question looming over Sanders is how much guidance he is willing to take from advisers encouraging more aggression against Biden — as the first debate indicated — and how much he will listen to others in the campaign favoring a less hostile approach. Unlike last time, when he was next to Biden, Sanders on Tuesday will be standing next to Warren, a friend and ideological ally whom Sanders and his aides have steered clear of directly criticizing. Warren has also had positive things to say about ­Sanders.

Some of their associates are not expecting that dynamic to change. Adam Green, a Warren supporter who co-founded the Progressive Change Campaign Committee, which is backing Warren, said he “would be surprised” if Sanders and Warren attack each other.

Warren signaled the same instinct Saturday after a town hall in Derry, N.H.

“Oh, Bernie and I have been friends forever, since long before I ever got into politics,” she said. “And I think the upcoming debate is a chance for everybody on that stage to talk about their vision for America. I know what I’m going to be talking about: a country that is working great for the wealthy and the well-connected, just not working for anybody else.”

Still, the tense dynamic between supporters of Sanders and Warren has been apparent in recent weeks. At a fundraiser Sanders held in Hollywood on Thursday night, Vic Mensa, one of the opening musical acts, made reference to “a candidate who in the ’80s was pretending to be Native American,” an apparent dig at Warren and her claims of Native American ancestry. Referencing a disparaging nickname given to her by Trump, a man in the crowd yelled out “Pocahontas!” He was quickly shushed by someone else.

Along with Warren and Sanders, Tuesday night’s debate will include Sen. Amy Klobuchar (D-Minn.), South Bend, Ind., Mayor Pete Buttigieg and former congressman Beto O’Rourke (D-Tex.).

Klobuchar’s campaign sees the debate as a chance to offer a prominent counterbalance to the leftward shift of the party. She has talked about the need for deadlines, not just plans — an implicit rebuke of Warren’s mantra of “I have a plan for that” — and she, like Biden, opposes the Medicare-for-all plan Sanders aggressively promotes.

O’Rourke struggled during the first debate; those close to him say he is attempting to be more clear-minded this time.

“He’s going to just be himself — not try to cram down all of the things that people are telling him he has to say,” said Steve Ortega, a longtime friend from El Paso.

Michael Scherer and Annie Linskey contributed to this report.