MIAMI — The early stages of Kamala Harris’s presidential campaign have been defined by peaks and plateaus, moments of strength that have inspired supporters and others that have confounded even her closest allies.
But in her first Democratic presidential debate this week, Ms. Harris, the barrier-breaking California senator who is seeking to become the first black woman elected president, fashioned her biggest breakthrough yet — an inspired performance in front of millions of viewers that could provide her campaign the clarifying moment it has long sought.
Ms. Harris’s debate night was a “game changer,” said Aimee Allison, founder and president of She the People, a political advocacy group focusing on women of color.
While Ms. Harris has been criticized for being overly cautious, her performance in Miami on Thursday night was anything but. She upended the debate as she directly challenged the early front-runner in the race, former Vice President Joseph R. Biden Jr., over his history of opposing school integration through busing. And a hush fell over the room when she said it was “hurtful” that Mr. Biden had warmly recalled his work with segregationist senators in remarks last week.
Ms. Harris, who has been reluctant to talk about her biography on the campaign trail, also invoked her personal history as a young girl who was bused to school as part of an integration program in California, and her campaign is now selling T-shirts designed with her childhood photograph. “That Little Girl Was Me” the T-shirts read.
A Harris aide said that Thursday was the campaign’s biggest day for online fund-raising since the 48 hours after Ms. Harris entered the race.
On Friday, Ms. Harris joined some of her 2020 rivals at a migrant detention center in Florida, a visit intended to show a united front against President Trump’s immigration policies. But the debate made clear that Ms. Harris is prepared to find ways to set herself apart from the pack, casting away any lingering doubts that her campaign was unwilling to draw direct contrasts with her Democratic opponents.
“Her words were a breathtaking acknowledgment of the importance and power of women of color who have long been dismissed and our rights and protections traded away even by those in the same party,” Ms. Allison said. “She called attention to the personal wounds and political tragedy of racism, a message every American needs to hear.”
Ms. Harris’s debate moment was emblematic of the promise of her entire campaign, which began at a rally in her hometown, Oakland, Calif., that drew more than 20,000 people. Though her poll numbers have been stuck in the high-single digits since she entered the race, she has been building toward a confrontation with Mr. Biden over the last month, making an increasingly overt bid for the moderate voters, and particularly older black voters, who form Mr. Biden’s strongest constituency.
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In recent campaign stops in Detroit and South Carolina, both hosted by the N.A.A.C.P., Ms. Harris has been putting more of her focus on attacking Mr. Trump and advocating for pragmatic change, sounding more like Mr. Biden and less like the progressive Democrats she was initially compared to, such as Senator Bernie Sanders of Vermont or Senator Elizabeth Warren of Massachusetts.
Her shift came after internal campaign tensions emerged in May, when Ms. Harris’s desire to appease her progressive critics created several small controversies. On multiple occasions, she seemed to express openness toward liberal policy stances — such as the prospect of incarcerated people voting from prison, and the elimination of private health insurance under a “Medicare for all” proposal — and then quickly backtracked.
Privately, Ms. Harris’s critics and rivals wondered if she was being pulled in opposing directions, by the progressive wing of the party that wants a candidate who openly embraces structural change and by her more natural instinct as a pragmatist. They also questioned her organizing strength in Iowa and New Hampshire, two key early voting states.
Ms. Harris has struggled to turn previous memorable moments into a sustained bump in polling, and political observers said again on Friday that her challenge was not in attracting interest from voters — which she already had — but in translating that interest into sustained support.
Ms. Harris’s team has stressed that her shift in recent months to more Trump-centric rhetoric was not a panicked reset of her campaign, but part of a natural evolution. They have cast the Miami debate performance as an attempt to show voters that she could take on Mr. Trump without fear.
In recent weeks, Ms. Harris has also picked up several new endorsements from members of the Congressional Black Caucus, including Representative William Lacy Clay of Missouri, Representative Al Green of Texas and Representative Alcee L. Hastings of Florida. On Friday, the campaign released statements from influential Democrats in early primary states, like the Rev. Dr. Ralph Brown, a pastor in Orangeburg, S.C.
“After watching her performance on the debate stage I know she can take on Donald Trump and win back the White House for working families,” Dr. Brown said.
But even in the afterglow of her debate reviews, questions still linger about Ms. Harris’s policy core. After her exchange with Mr. Biden on Thursday night, a Harris spokesman said that she supported busing as a method for school integration, but the campaign declined to provide additional information. Busing has, for decades, been a controversial education policy that has been panned by white voters, even in liberal urban centers.
The debate also did little to clarify Ms. Harris’s plans for improving the nation’s health care system. When the candidates were asked to raise their hands if they supported getting rid of private health insurance to create a single-payer system, Ms. Harris was one of two candidates, along with Mr. Sanders, who did so. Later, Ms. Harris said she had misunderstood the question. It was the second time this year she has had to further clarify her stance on single-payer health care, which has become a key dividing line among Democrats.
“Do you believe private insurance should be eliminated in this country?” she was asked in a follow-up interview on MSNBC on Friday morning.
“No, I do not.” Ms. Harris responded. “The question was, ‘Would you give up your private insurance for that option?’ and I said, ‘yes.’”
At Ms. Harris’s campaign events, it is rare for voters to raise such policy issues. At this early stage of the race, when people often come to events for a first introduction to a candidate, there is a palpable sense of expectation that surrounds Ms. Harris, as Democrats arrive hoping to see the black woman who could debate Mr. Trump.
But Ms. Harris can still be uncomfortable talking about herself in those settings, said Rashad Robinson, the president of the civil rights advocacy group Color of Change, who is close with the senator and her inner circle.
Ms. Harris needs to “share more of the why,” he said. “Why is she running? Who is she? I think folks are craving connection.”