WASHINGTON, July 22: After intense pressure from within his own party, President Biden said he was ending his campaign and backing Vice President Kamala Harris to run in his place. Ms. Harris said she would seek the nomination, adding: “Together, we will fight. And together, we will win.”

Powerful leaders of the Democratic establishment quickly embraced Vice President Kamala Harris on Sunday after President Biden’s shocking exit from the race, hoping that a seamless succession could end a month of damaging chaos and transform a contest widely believed to be tipping toward Republicans.

By Sunday evening, Ms. Harris appeared to have a glide path to the nomination: No other top Democrats announced plans to challenge her, though some stopped short of an endorsement, including the party’s top congressional leaders and former President Barack Obama.

With breathtaking speed, she took control of Mr. Biden’s enormous political operation and contacted Democratic leaders in Congress and state houses to ask for their support. The Biden campaign formally renamed itself “Harris for President,” giving her immediate access to an account that had $96 million in cash at the end of June. On an internal call, the Biden campaign’s leaders told staff members that they would now work for Ms. Harris.

“I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump and his extreme Project 2025 agenda,” Ms. Harris said in a statement. “We have 107 days until Election Day. Together, we will fight. And together, we will win.”

The rapid turn of events plunged the party and the nation into unfamiliar political territory, giving unelected Democratic officials the final say over the party’s nominee. Complicated decisions loom. Ms. Harris must choose a running mate, take charge of the campaign with little time before early voting begins in some states in September, rebuild support among voters who had fled Mr. Biden and prepare to withstand a full-blown Republican assault.

Speculation immediately turned to her potential running mate, with many Democrats privately arguing that Ms. Harris should pick a white man to widen her appeal and provide demographic balance to the ticket. A flotilla of governors — including Roy Cooper of North Carolina, Andy Beshear of Kentucky, Josh Shapiro of Pennsylvania and Tim Walz of Minnesota — as well as Senator Mark Kelly of Arizona have been frequently mentioned by donors, officials and other lawmakers.

Mr. Biden endorsed Ms. Harris within minutes on Sunday afternoon. Before she had uttered a word about her intentions, he was swiftly followed by other party leaders, including Bill and Hillary Clinton, key Democratic senators and influential House members. By the evening, Mr. Shapiro had publicly backed Ms. Harris after a phone call from her. She also picked up endorsements from Gov. Gavin Newsom of California, Mr. Cooper and Mr. Kelly.

Not all Democrats were pushing for a coronation. Neither Senator Chuck Schumer, the majority leader, nor Representative Hakeem Jeffries, the minority House leader, backed her, nor did Representative Nancy Pelosi, the former speaker who wields considerable power. Mr. Obama appeared to call for a more competitive nominating process.

“We will be navigating uncharted waters in the days ahead,” he said. “But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.”

Privately, some Democrats argued that a more competitive process would benefit Ms. Harris because it could dispel doubts about her political skills. Some also said it could counter perceptions that the party was making decisions for the country in today’s equivalent of a smoke-filled room. Ms. Harris said on Sunday that she intended “to earn and win this nomination.”

The party’s quick embrace of Ms. Harris poured a dash of cold water on the idea that some other challenger would emerge. Any Democratic rival would face a steep climb to match her political support, money and other resources. Such a bid could also hurt the candidate’s future prospects, particularly among Black women, who are some of the party’s most reliable voters.

Many Democrats felt a palpable sense of enthusiasm and relief. They now see an opportunity to recover from nearly a month of internal battles and a race that seemed headed for catastrophic defeats from the White House all the way down to statehouse races.

“Democrats have a choice to make: unite and win, or fight each other and lose,” said Mayor Randall L. Woodfin of Birmingham, Ala., urging his party to embrace Ms. Harris. Mr. Woodfin, who had been a strong supporter of Mr. Biden’s, said Democrats now had “a chance to have a younger ticket that pulls in more younger voters, enthusiasm, energy and excitement.”

While Ms. Harris may inject enthusiasm into the Democratic grass-roots, it remains uncertain how the country will respond to her campaign. She is a Black woman from California, a liberal stronghold, and her bid will inevitably test the country’s appetite for a historic first.

In 2020, Democratic primary voters rallied around Mr. Biden because he appeared to be the safest choice against Mr. Trump. Now, after Mr. Biden’s disastrous debate performance, the party appears ready to embrace a candidate many still believe could face significant headwinds.

 
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