It was when, as others talked about racial issues, that she broke in and changed the flow of the evening by reminding voters of the attributes that help define her candidacy. “As the only black person on this stage, I would like to speak on the issue of race,” she declared.

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Biden asserted that Harris’s criticisms on race were a “mischaracterization of my position across the board.” But her attacks, delivered at close range and with forcefulness and personal references, left a mark on the former vice president and established her as a candidate to be reckoned with.

Harris has often struggled to match the promise of her candidacy, but in her first opportunity arrayed as one of 10 candidates, she made the most of the opportunities she was given — and took some on her own to announce her arrival on the big stage.

Thursday’s debate may not change the polls much but it will probably reorder how Democrats begin to think about the choices before them.

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The differences between Biden and Sanders, which are real and unresolved, proved to be only a subtext of the Thursday debate. Instead it was other candidates who helped drive the debate and shape the tone.

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Sen. Michael F. Bennet (Colo.) came close to Harris at times, whether attacking Biden for cutting a deal that made permanent the tax cuts first passed during the presidency of George W. Bush or offering counterarguments to those candidates advocating a single-payer Medicare-for-all program that would eliminate private insurance.

Biden was the focus of a series of criticisms by his rivals, but it was Harris who braced him most directly when she challenged him for his recent comments about working with two segregationist senators decades ago and for opposing school busing, noting that she was a beneficiary of busing policies. She also pointed to her differences with the Obama-Biden administration on the deportation of undocumented immigrants.

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Biden is an experienced debater and knew how and when to parry the attacks that kept coming at him. But Thursday’s debate underscored what many Democrats have said about this nominating contest, which is that, while Biden is the clear leader in the polls, he is not the kind of dominating candidate that others who have enjoyed the label of front-runner have proved to be.

Hours before the debate, Biden campaign advisers said he would use the evening to talk about “really transformational change” by stressing his belief that he can end this period of hyper-partisanship and return the country to civility and consensus. But he rarely got the opportunity to make that case. Instead he spent the evening on the defensive.

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The bracing attack by Harris proved to be the most dramatic moment of either of the two nights of debating among the 20 candidates who qualified under Democratic National Committee rules to be on the stage in the Adrienne Arsht Center.

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Her opening came after South Bend, Ind., Mayor Pete Buttigieg was asked about the racial turmoil in his city stemming from the recent killing of a black man by a white police officer . Buttigieg delivered a heartfelt answer to a pointed question about why the force was barely integrated in a city that is a quarter African American.

“I could walk you through all of the things that we have done as a community, all of the steps that we took, from bias training to de-escalation, but it didn’t save the life of Eric Logan,” he said. “And when I look into his mother’s eyes, I have to face the fact that nothing that I say will bring him back.”

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Buttigieg said the issue of racism and policing “threatens the well-being of every community” and vowed to continue to help find solutions locally and nationally. The discussion moved to other candidates until Harris broke in.

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After words of introduction on the topic of race, she turned to Biden and the former prosecutor began to make her case. “I do not believe you are a racist, and I agree with you when you commit yourself to the importance of finding common ground,” she said. “But I also believe, and it’s personal . . . it was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country.”

She continued: “And it was not only that, but you also worked with them to oppose busing. And, you know, there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me.”

Earlier in the debate, as the conversation was spiraling out of control and the moderators were struggling to maintain order, Harris shut them all down with a quip that, even if prepared in advance, accomplished her goal of putting the focus on her.

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“Hey, guys, you know what? America does not want to witness a food fight,” she said. “They want to know how we are going to put food on their table.”

Harris staked out positions that put her clearly in the liberal wing of the party, particularly on health care, positions that Republicans believe would make her vulnerable. But her goal on Thursday was not to persuade Republicans but to awaken Democrats to her candidacy’s potential, and on that she succeeded.

Biden retains the goodwill of many Democrats, including African Americans, and continues to make the case that he is best positioned to defeat President Trump. But his performance probably will raise questions about his candidacy.

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Sanders, who defended his label of democratic socialist with typical confidence, retains a loyal following and will remain a force in the months ahead.

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But Bennet emerged as a voice prepared to challenge the progressive wing, in the way that Biden is expected to do. And Harris has put her stamp on her candidacy far more effectively than she had done before Thursday’s contest.

As on Wednesday, Thursday’s debate highlighted the ideological tensions within the Democratic coalition, fissures that will dominate the nomination contest well into next year. Those tensions will test the priorities and loyalties of Democratic voters as they select their challenger to go against Trump in the general election.

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Some party strategists say their soundings so far this year suggest a streak of pragmatism among the Democratic electorate. They argue that many voters who would prefer a robustly liberal agenda are prepared to set aside some of those ambitions in favor of a candidate with whom they might disagree on some issues but believe has a better chance to deliver the White House in November 2020.

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Biden on the one hand, and Sanders and Sen. Elizabeth Warren (D-Mass.) on the other, seemed to represent the two poles in that debate, until the Democrats came to Miami. Now the choices seem to have widened, with new and younger voices rising to make themselves heard.

The first round of debates is now in the archive. Some moments on the highlight reels from both nights will live through the weekend and into the next week but probably will have a limited shelf life. Few anticipated that the debates would significantly alter the shape of the race, and perhaps that is the case.