Throughout her history-making and often rocky ascent to the summit of American public life, Kamala D. Harris has shown an uncanny ability to revive her political fortunes.

Now she is on the verge of a more momentous test: With just over 100 days before the presidential election, can she revive the fortunes of the Democratic Party?

If Democrats choose Harris to replace President Bidenatop the ticket, as Bidenasked them to dowhen he announced his withdrawal from the raceon Sunday, she would become the first Black woman, and first person of South Asian descent, to be nominated for the presidency by a major party. It wouldrepresent a remarkable comeback for a politician who not long ago seemed destined to join the long list of promising state elected officials who flame out on the national stage.

Harris stands to become the leader of a party in crisis. Democrats are scrambling after Biden’s unprecedented decision to decline the nominationonly 15 weeks before the general election, with polls showing former president Donald Trump leading in key battleground states. Harris, like Biden,has an approval rating that has not cracked 40 percent this year — a worrisome sign that she may still be carrying the administration’s electoral baggage.

On Sunday night, it was still unclear whether the party would coalesce around her as smoothly — or quickly — as she or Biden hope. In recent days, as Biden’s position became less tenable, some Democrats have urged an open nomination process rather than a coronation of the vice president. After Biden’s announcement, Harris said in a statement that she was “honored to have the President’s endorsement” and that “my intention is to earn and win this nomination.”

Time is short: Democrats convene in Chicago next month for their convention, but plan to formally nominate their ticket by virtual roll call in early August.

If Democrats do elevate Harris as their nominee, they willbe placingan extraordinary bet that she is finally ready to deliver on her promise and overcome her uneven history as a campaigner. She would have less than four months to win what could be one of the most consequential elections in American history, and would have to do so against an opponent willing to stoke sexism and racism to win votes — and who is riding a new wave of adulation within his party after surviving an assassination attempt this month.

However events unfold, Harris and the Democrats are in uncharted territory, said Russell Riley, a presidential historian at the Miller Center at the University of Virginia. Former president Lyndon B. Johnson’s decision not to seek reelection in 1968 — a precedent some have cited in urging Biden to drop out — was announced just over seven months before the general election, setting up a timeline that was languid by comparison.

“There is no direct historical analogy that I’m aware of,” Riley said.

Historically unprecedented battles are not new to Harris, who has defied expectations, for good and ill, since her earliest days in politics.

A former prosecutor who emerged from the ruthless world of San Francisco politics, Harris was once compared to former president Barack Obama. She was written off after her disastrous 2019 presidential bid but got a reprieve with her selection — and largely successful performance — as Biden’s running mate. After the election came a string of unflattering headlines about her alleged mismanagement of the vice president’s office and her sometimes maladroit communication of the president’s agenda, most notably on immigration.

Yet in the wake of the Supreme Court overturning Roe v. Wade two years ago, Harris again seemed to find her footing, returning to her roots in the courtroom as she became the Biden administration’s most effective advocate for women’s reproductive rights.

And as Democrats began to panic in June over their 81-year-old nominee’s infirmity after a catastrophic debate performance against Trump, many began to speak of Harris as a practical and perhaps exemplary alternative — a 59-year-old woman prepared to energetically press the case against theformer president.

Bill Carrick, a veteran Democratic strategist from California who worked for Harris’s principal opponent in her 2016 Senate race, said that watching the vice president barnstorm the country as a Biden campaign surrogate in recent months, he has noted a remarkable improvement.

“She’s a better candidate now than she was either as district attorney or attorney general or the presidential candidate from 2019,” he said. “She’s exhibiting a lot of personality and a lot of substance. It’s pretty dramatic.”

Ashley Etienne, former communications director for both Harris and Rep. Nancy Pelosi (D-Calif.), said that if Harris becomes the nominee she is “almost going to have a clean slate” and needs to make the most of the coming months to tell her story to voters.

“Think about the person who is only consuming Fox News,” Etienne said. “Some portion of those people you have to peel off, so she’s going to have to do some introduction and really lay out her own vision.”

The Trump campaign has already begun its efforts to tie Harris to Biden, writing in a statement on Sunday, “They own each other’s records, and there is no distance between the two. Harris must defend the failed Biden Administration AND her liberal, weak-on-crime record in CA.”

The daughter of an Indian mother and Jamaican father, Harris was born in 1964 and grew up in Berkeley, Calif. After attending Howard University and the University of California Hastings College of the Law (now known as the University of California College of the Law at San Francisco), she worked as a prosecutor of sex crimes, homicides and other serious offenses in Alameda County and San Francisco.

It was a fateful course for her political identity. In 2003 Harris challenged her old boss, San Francisco District Attorney Terence Hallinan, a leftist lion who, before winning office as the city’s top law enforcement official, had made a name defending countercultural figures such as Anton LaVey, the founder of the Church of Satan.

In a three-way race, Hallinan came out ahead in the first round of voting, but Harris came in second and handily won the ensuing runoff election. She won reelection uncontested in 2007, and not long afterward began preparing to run in the 2010 race for attorney general. Her opponent was Los Angeles District Attorney Steve Cooley, a Republican.

At a time when the GOP’s statewide electoral prospects in California were not quite so dim, the race was a nail-biter. Harris trailed in the final weeks of the campaign, and as votes were counted on election night Cooley prematurely declared victory. By the next day, as the tallying continued, Harris gained the lead, and after several weeks of counting mail-in and provisional ballots, she won by a less-than-one-point margin.

Harris spent six years as the Golden State’s top law enforcement official, focusing on the mix of consumer-protection issues and complex criminal investigations that make up a state attorney general’s portfolio. After longtime DemocraticSen. Barbara Boxer announced her retirement in 2015, Harris stepped into the race as the favorite to succeed her, and without strong opponents she easily won.

As the junior senator from California, she began to gain a national profile — particularly whenshe channeled Democratic outrage at the Trump administration in questioning his nominees and members of his administration. Trump’s attorney general at the time, Jeff Sessions, memorably remarked that Harris’s questions “make me nervous.”

Three years after entering the Senate, she joined more than two dozen Democrats vying for the presidential nomination. Harris started strong, drawing more than 20,000 people to her campaign launch in front of City Hall in Oakland, Calif.

She later drew attention when she chided Biden during a debate for talking about compromises he had made with segregationist senators.

“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,” Harris said, in what became the most memorable line of her campaign. “And that little girl was me.”

But despite her promising start, she struggled to communicate a coherent message that resonated with voters and her bid eventually flatlined. She ran out of cash and withdrew from the race in December 2019.

Some veterans of that campaign still have deep misgivings about Harris, recalling a candidate who refused to prepare for events, was often plagued by indecision and would frequently lash out at staff.

“It was the most frustrating period of my life as a political staffer,” one former aide recently said, speaking on the condition of anonymity to discuss internal campaign dynamics.

Biden, the primary’s eventual winner, had vowed to pick a woman as his running mate. Amid calls for racial equality after the killing of George Floyd by police, Biden’s allies stressed that he pick not just a woman, but a Black one.

Eight months after her campaign for the White House had ended, and in the throes of the coronavirus pandemic, Biden announced Harris as his VP choice. The selection had personal undertones as well as political: Harris was a friend and colleague of Biden’s late son, Beau, when both were attorneys general.

It was a successful ticket against Trump, but Harris, as vice president, got off to a rough start. She at times struggled in interviews, such as when she offered a garbled response when NBC’s Lester Holt asked whether she would visit the Southern border to address illegal immigration.

By the time she reached the one-year mark, her office had also been rocked by a series of staff departures, including her chief of staff, her chief spokeswoman and her communications director. The resignations reignited questions about why Harris churns through top-level Democratic staff, an issue that has dogged her for almost all of her time in public service.

To regain her footing, she brought on Lorraine Voles as chief of staff. Voles had served in senior roles with Vice President Al Gore and Sen. Hillary Clinton. And observers began to notice achange two years ago, when Harris homed in on conservatives’ efforts to strip away abortion access after the Supreme Court’s Dobbs decision.

Biden, a lifelong Catholic, has struggled to forcefully articulate his party’s position on abortion. Harris, by contrast, had defended abortion access throughout her career and was well-positioned to take up what many Democrats now saw as a winning issue.

As he stepped down from the top of the ticket Sunday, Biden tacitly acknowledged his conclusion that Harris is now better poised than he — or anyone else in the party — to meet the moment in a vital confrontation with Trump.

“My very first decision as the party nominee in 2020 was to pick Kamala D. Harris as my Vice President. And it’s been the best decision I’ve made,” Biden said in a statement on X. “Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it’s time to come together and beat Trump. Let’s do this.”

Ashley Parker and Josh Dawsey contributed to this report.