Here’s what else to know:

• Biden backs Harris: Mr. Biden gave Ms. Harris his “full support and endorsement” in a social media post. “Democrats — it’s time to come together and beat Trump,” he added. “Let’s do this.” Ms. Harris said she was “honored” by Mr. Biden’s endorsement and that her intention was to “earn and win” the party’s nomination. “Together, we will fight, she said in a statement. “And together, we will win.” After getting off to a rocky start as vice president, Ms. Harris now stands at the brink of leading her party’s ticket.

• A sudden withdrawal: Mr. Biden’s decision ends a political crisis that began when the president delivered a calamitous debate performance against Mr. Trump on June 27. For weeks, the president insisted that he would remain in the race, but a senior administration official familiar with Mr. Biden’s thinking said he changed his mind in part because he had tried for weeks to flip the attention back to Mr. Trump. Here’s a timeline of Mr. Biden’s decision to drop out.

• Uncertainty looms: No sitting American president has dropped out of a race so late in the election cycle. The Democratic National Convention, where Mr. Biden was to have been formally nominated by 3,939 delegates, is scheduled to begin Aug. 19 in Chicago, and Democrats are gaming out the scenarios for a new nominee, even if Ms. Harris has certain built-in advantages — including with the “Biden for President” campaign committee officially filing paperwork to rename itself “Harris for President.” One crucial question: What happens to the $96 million already in the Biden campaign’s coffers? It seems likely that Ms. Harris can inherit it.

• Harris picks up endorsements: Mr. Biden’s endorsement of Ms. Harris set off a flood of support from his fellow Democrats. Among them: former President Bill Clinton; Hillary Clinton, the former secretary of state and Democratic nominee in 2016. Governors Gavin Newsom of California and Josh Shapiro of Pennsylvania, both of whom had been seen as potential contenders themselves, also backed her, as did a number of high-profile lawmakers, including the leaders of three major power centers on Capitol Hill: the Progressive Caucus, the Congressional Black Caucus and the centrist New Democrats.

• Important Democrats mum on Harris: Four of the party’s most influential figures issued statements praising Mr. Biden, but stayed silent on Ms. Harris, including former President Barack Obama, who was said to be maintaining the neutral stance he took during the 2020 primaries. While many Democrats on Capitol Hill rallied around Ms. Harris, Senator Chuck Schumer and Representative Hakeem Jeffries, the New York Democrats who lead the party in Congress; and Representative Nancy Pelosi of California, the former House speaker, stayed silent. Read about the reaction in Congress.

• Donors and voters react: Billionaires and major donors quickly flocked to Ms. Harris, while Democratic voters responded to Mr. Biden’s exit with a mix of relief and optimism. The leading site processing Democratic donations, ActBlue, raised more than $50 million for Democrats on Sunday, according to a New York Times analysis. It was the single biggest day for online Democratic contributions since the 2020 election. Here’s a look at how the vice president polls against Mr. Trump.

• Praise for Biden: After weeks of deep concerns about his age and ability to win again roiled the party, Democrats hailed Mr. Biden’s accomplishments as president. Ms. Harris praised his “big heart,” while four governors who have been mentioned as possible nominees applauded his legacy. Mr. Biden’s son Hunter said that “unconditional love” had been his father’s “North Star.” Jill Biden, the first lady, responded to her husband’s statement with only a heart emoji. Here’s a look at what will be Mr. Biden’s legacy.

• Trump reacts: Mr. Trump seized on the moment to criticize Mr. Biden, saying he was never fit to be president, and denounced the Washington political establishment and the news media, saying they “did everything they could to protect” him.

Vice President Kamala Harris swiftly established herself as the Democratic front-runner to take on Donald J. Trump within hours of President Biden’s exit on Sunday, fundamentally rewiring the presidential contest at warp speed.

Now the race has been transformed into an abbreviated 106-day sprint that more closely resembles the snap elections of Europe than the drawn-out American contests. The tight timeline will magnify any missteps Ms. Harris might make but also minimize the chances for a stumble.

And in a race that Mr. Trump had been on a trajectory to win, Ms. Harris immediately becomes the ultimate X-factor.

Mr. Biden quickly endorsed Ms. Harris, who would be a barrier-breaking nominee as the first woman, the first Black woman and the first person of South Asian descent ever to serve as president. As the Democratic Party rallies behind her — the loudest voices of dissent were simply those not publicly endorsing her — here are six ways her candidacy holds both promise and peril.

She inverts the age argument.

During the Republican primaries, Nikki Haley had warned everyone who would listen that the first party to swap out its octogenarian candidate — Mr. Trump will turn 80 while in office if elected to a second term — would win. She was making the argument for herself but the logic applies to Ms. Harris, too.

Unlike the 81-year-old Mr. Biden, Ms. Harris, 59, is not old — and just that fact neutralizes what has been one of the most potent Trump lines of attack.

Within minutes of Mr. Biden’s quitting, Democrats and anti-Trump Republicans were questioning Mr. Trump’s capacity to govern into his 80s, a daring attempt to reframe an age debate that has been so damaging to Democrats.

“She can make the issue of age and fitness a liability for Trump,” Erin Wilson, Ms. Harris’s deputy chief of staff, said on a call on Sunday with the group Win With Black Women.

Polls have consistently shown that voters have not been overly concerned with the 78-year-old Mr. Trump’s age. But simply taking the issue off the table may be enough of a victory for Democrats. They were facing the stiff headwinds of three-quarters of Americans thinking Mr. Biden was too old — a view shared widely even before his doddering debate.

Ms. Harris is also expected to give Democrats a far more vigorous campaigner. Her day job is not nearly as demanding as Mr. Biden’s, and she can barnstorm the country at a pace far faster than Mr. Trump has undertaken.

She’s a former prosecutor. Trump’s a convicted felon.

Ms. Harris has often been at her best politically when she has taken on the role of prosecutor-in-chief, whether on the debate stage when she first bore into Mr. Biden in June 2019 over busing or as a senator on the Judiciary Committee where her intensive cross-examinations went viral.

When she ran for president, among her tag lines — and her struggling campaign cycled through a few catchphrases — was that she was best positioned to “prosecute the case” against Mr. Trump.

Now she will have the chance to do so in the same year in which an actual prosecutor in New York scored 34 felony convictions against him and Mr. Trump still faces more than one future criminal trial.

People who have worked with Ms. Harris believe that framework could allow her to play to some of her strengths — and expose some of Mr. Trump’s weaknesses. Polls have shown a noteworthy share of voters think Mr. Trump has committed crimes yet were still planning to vote for him.

Biden was ‘Scranton Joe.’ Harris will be tagged a California liberal.

If Mr. Biden was widely seen as too elderly to lead, he had other advantages built up over 50 years in the public spotlight. Namely, he has long been viewed as a more moderate Democrat who pushed back against the more extreme elements of his party. It helped him appeal to the political middle.

“Do I look like a radical socialist with a soft spot for rioters?” he fumed at one point in the 2020 race. His image was such that at times Republicans opted to attack him by suggesting he was being directed by other forces.

Ms. Harris does not have that advantage.

Instead, Ms. Harris got her start in politics as the district attorney of one of the nation’s most famously liberal cities, San Francisco, before winning statewide in one of the nation’s most famously liberal states, California. (Mr. Trump, notably, was among her donors then.)

And while Ms. Harris did not carve out a reputation in California as an outspoken progressive — her tagline as D.A. was about being “smart on crime” — when she ran for president in 2020 she regularly staked out positions to Mr. Biden’s left, including embracing a “Medicare for all” system that he had avoided.

As Mr. Biden’s partner for the last three-and-a-half years, Ms. Harris faces the added burden of supporting the agenda of a president who has become deeply unpopular.

The Trump team has already signaled they plan to attack her on immigration in particular. The question is whether Ms. Harris can successfully find a way to campaign on some of the Biden-Harris administration’s most popular accomplishments without the current unpopularity of the man who previously led the ticket.

She gives Democrats a much-needed jolt of momentum.

Mr. Trump and his advisers were not looking to shake up a race he was winning by almost every metric. As Republicans gathered last week in Milwaukee, they were downright jubilant about the direction of 2024, seeing Mr. Trump as almost a candidate of destiny days after he had survived an assassination attempt.

Now his team must shift to run a very different race against a very different candidate. Ms. Harris has the ability to potentially energize the Democratic base — especially some of the core constituencies who had felt alienated — in ways Mr. Biden no longer seemed capable of. The president had struggled, relative to his 2020 performance, among Black voters and younger voters in particular, constituencies that Ms. Harris’s historic potential candidacy would seem poised to improve upon.

In an early sign of the Democratic appetite for a change, donors contributed more than $60 million online on Sunday — the third biggest day in the history of ActBlue.

It was also notable that Mr. Trump cast doubt on a future debate with Ms. Harris after he had so eagerly sought to share a stage with Mr. Biden, suggesting a venue change from ABC to Fox News.

Her gender could galvanize Democrats — and also Republicans.

In the 2020 primary, Democratic voters wrestled for months with the question of who would be the strongest candidate against Mr. Trump. They wondered, often aloud, about the idea of nominating a woman.

Mr. Trump, after all, had just defied expectations and defeated Hillary Clinton in 2016. The party ultimately selected an older white man in Mr. Biden.

For much of the Trump presidency and beyond, Democrats have benefited from a gender gap. Women voted for Democrats by a bigger margin than men favored Republicans. But Mr. Trump has swelled his advantage so much among men of late that the gender gap has been suddenly favoring the G.O.P.

Ms. Harris has a chance to reverse that and has already proved herself a far more compelling messenger than Mr. Biden on the issue that Democrats believe can win them the 2024 race. Mr. Biden rarely would say the word abortion; Ms. Harris visited an abortion clinic.

Ms. Harris faces other distinctive challenges as a Black candidate and a woman, in a country and a political system where both groups are often held to different standards. And in Mr. Trump, she faces an opponent with a history of exploiting stereotypes for his own advantage.

She can be transcendent, but also tentative.

One of the notable facts of Ms. Harris’s speedy rise to the pinnacle of Democratic politics in a little more than a decade is how few loyalists have been along for the full ride.

If Mr. Biden surrounded himself with a small, sometimes insular, coterie of advisers — a recent Biden inner-circle addition could have served him for a decade — Ms. Harris has relatively few similarly long-standing aides. Early on as vice president, her staff turned over significantly.

She has few advisers dating back even to her days in the Senate, let alone her time as attorney general of California. She parted ways with a swath of the senior team on her 2020 presidential primary run, which was wracked with infighting.

Those who have worked both for and against her say she has few equivalents when she nails a big speech, or delivers an acerbic line on the debate stage or a committee hearing. But they also say she can get in her own head, retreat back to canned comments and make tentative, self-inflicted mistakes.

Now she is inheriting Mr. Biden’s enormous campaign apparatus. And she has little more than 100 days to capture both the Democratic nomination and the presidency. source