Early Monday morning, Biden’s campaign learned that Sen. Amy Klobuchar (D-Minn.) would also be dropping out and was eager to endorse the former vice president. Soon, Buttigieg confirmed he would do the same.

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The rush to organize the day was interrupted when an alarm went off in Biden’s Philadelphia headquarters. A gas leak forced an evacuation, and some staffers left their laptops behind. They eventually decamped to a common space in an apartment building, turning on televisions to keep themselves in the loop.

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Then, that afternoon, they learned former candidate Beto O’Rourke was ready to tell his fellow Texans that he planned to vote for Biden — and was willing to immediately hop on a plane to Dallas to attend a Biden rally that evening.

Those hours brought a chaotic cascade of good news for Biden’s presidential effort and a mad scramble to maximize a sense of drama and momentum that Biden’s campaign had rarely, if ever, enjoyed.

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They were a key part of a three-day span that utterly remade the Democratic race and set in motion a political comeback without precedent in modern presidential politics.

Much had set the stage for those days. Former New York mayor Mike Bloomberg stumbled in debate performances that effectively took him out of the race. Many in the party grew alarmed at the damage they saw it suffering if Sen. Bernie Sanders (I-Vt.) were the nominee.

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Yet the depth of Biden’s win in South Carolina showed his continued strength among black voters, a key Democratic constituency, and renewed power among rural and suburban voters. More than that, it redefined Biden as a candidate who could win.

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Lagging before the South Carolina primary, he would romp to 10 wins on Super Tuesday. The victories blunted Sanders’s effort to build an insurmountable delegate lead.

Instead, it was Biden who would emerge with a lead in delegates. The developments were driven by the colliding imperatives of cold numerical realities and the personal stakes of running for the highest office in the land. And it occurred with breathtaking speed.

“This is really hard. It’s really hard giving up the dream, but it was the right thing to do,” said Justin Buoen, Klobuchar’s campaign manager.

On Saturday night, in Raleigh, N.C., Buttigieg looked at the numbers coming out of South Carolina. Biden’s victory was so surpassing in exit polls that he was declared the winner seconds after the polls closed. Buttigieg had not made up ground with the party’s black voters, who remained intensely loyal to Biden, and he no longer had a path forward.

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He took the stage for what would probably be his last campaign rally and opened by saying: “Running for president is an exercise in hope and in humility.”

He ended by thanking the crowd as Creedence Clearwater Revival’s “Up Around the Bend” began to play — then the famously disciplined candidate grabbed the microphone and told the crowd he was going “off-script” to answer some questions read by Reggie Love, a former assistant to President Barack Obama and a supporter.

Eventually he ended the rally for a second time. His husband, Chasten, joined him onstage for a hug that seemed longer than usual.

Later that night, Buttigieg called his top advisers to let them know he didn’t want to linger in the race. They did not know what other candidates, like Klobuchar or Sen. Elizabeth Warren (D-Mass.), might do, but wanted to move quickly.

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“We, like other campaigns, saw that Senator Sanders was really going to consolidate and come out of Super Tuesday with an insurmountable lead” if the moderate candidates stayed in the race and fractured the vote, said Buttigieg campaign manager Mike Schmuhl. “That was too much of a risk.”

Klobuchar was in Charlotte, running through the same calculations. Internal polls showed she probably would win her home state of Minnesota on Tuesday but seemed unlikely to win anywhere else.

She could either stay in the race, give a victory speech at home and then drop out a few days later — or get out before Super Tuesday and put her support behind another candidate.

On Sunday morning, all the major candidates except Sanders gathered in Selma, Ala., to commemorate “Bloody Sunday,” the 1965 attack on marchers on the Edmund Pettus Bridge, a seminal event in the civil rights struggle.

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At a service at the historic Brown Chapel AME Church, Biden was welcomed like a close friend and given a seat of honor on the dais.

Bloomberg, who spent millions on advertising and organizing in the state, sat in the first pew and was invited to address the congregation. As he did so, 10 people stood and turned their backs in silent protest of his past police policies.

The Rev. Al Sharpton rose to declare that black voters “are not looking for better slave masters, we are looking for freedom.” Then Biden got up to speak and directly addressed Sharpton’s words.

“Where I come from, we’re not looking for masters. We’re looking for servants. That’s how I was raised,” Biden said. “So don’t get confused, old buddy. I was raised to be a servant. Not a master.”

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After the service, several candidates gathered to march across the bridge just as black activists had done 55 years earlier.

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“We must go out and vote like we’ve never, ever voted before,” said Rep. John Lewis (D-Ga.), whose skull was fractured in the 1965 march and who recently announced he has advanced cancer. “And help redeem the soul of America.”

The emotional scene gave Klobuchar time to reflect.

“I was thinking: What is better here? And what’s better for our country?” she said in a “Today” show interview. “Everyone knew this was the right thing to do.”

She looked over at Buttigieg, who appeared choked-up, and wondered if he also was thinking about dropping out.

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Unknown to her, he had already started calling his supporters to let them know he was doing just that. He and Biden would finally connect Sunday evening, after the news became public. Buttigieg also consulted with former president Obama.

That evening, Klobuchar arrived in the Minneapolis suburbs for a Sunday night rally — but was prevented from taking the stage by dozens of demonstrators who questioned the conviction of Myon Burrell, a black teenager who received a life sentence while Klobuchar was the Hennepin County district attorney. They chanted: “Free Myon” and “Klobuchar has got to go.”

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It was a blunt reminder of the problems Klobuchar had connecting with black voters and the pushback she would continue to face if she stayed in the race. Klobuchar canceled the rally.

On a chartered flight to Utah that night, she decided she would drop out the next day and endorse Biden. She needed time to implement her plan, so she showed up as scheduled to a campaign rally early Monday morning in downtown Salt Lake City, knowing it would be her last.

“The fact that I am still standing here is quite a tribute to grit,” she said wryly, lacking her usual energy.

Her voice caught with emotion as she asked the crowd “to understand that what unites us as a party and as a country is so much bigger than what divides us.” Standing alone on the stage, she waved goodbye, took a small bow and set down her microphone — then picked it back up.

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“I’m going to go,” she said, “but I want to thank you.”

An aide was already in touch with the Biden campaign. In the hours that followed, she would tell her staff and key supporters and reroute her plane to Dallas for a concession speech that doubled as an endorsement speech.

In Philadelphia, Biden’s campaign team worked to create a sense of momentum with a drumbeat of endorsements that had come his way after South Carolina.

“The campaign was laying out how important it was for a moderate to win on Super Tuesday,” said Rep. Tim Ryan (D-Ohio), who dropped out of the race and endorsed Biden last year. “It was made clear that if we were divided on Super Tuesday, it’d be messy.”

The scramble on Monday afternoon brought a better kind of messiness.

Biden’s campaign ordered a chartered flight to bring Buttigieg to Dallas to meet Biden at a hipster fried chicken joint. His aides decided Klobuchar would join Biden onstage at the later rally there, then fly to New York for morning-show interviews. The two endorsers, who had bitterly clashed on debate stages, would not be forced under the same spotlight.

Earlier that day, Biden reached out to O’Rourke, who had told people over the weekend he planned to vote for Biden on Tuesday because he was impressed by how Biden consoled the husband of a mass shooting victim during a televised town hall in South Carolina. O’Rourke was hesitant to announce his choice, but his eldest son urged him to help Biden get elected.

“Like a lot of other voters in Texas, I decided within the last week,” O’Rourke said.

By Monday afternoon, O’Rourke was scrambling to the El Paso airport to catch a last-minute flight to Dallas, purchased at his own expense. Landing about an hour before the rally was originally set to begin, O’Rourke took a taxi to the venue, where he was held backstage. Biden’s team decided to keep O’Rourke’s endorsement a secret until the rally’s final minutes.

As O’Rourke was en route, the former vice president offered Buttigieg high praise after his nationally televised endorsement, comparing him to his dead son Beau.

With Biden preparing to take the stage in Dallas, Rep. Ilhan Omar (D-Minn.) warmed up the crowd at a Sanders rally in St. Paul, Minn.

“While others are gathered tonight to fight our movement,” she said, “we are gathered to fight for somebody we don’t know.”

Sanders’s campaign had expected the moderates in the race would eventually unite against Sanders — but they were surprised it was happening so quickly.

Onstage in Dallas, Klobuchar delivered an emphatic endorsement of Biden. After the former vice president spoke, he called O’Rourke to the stage, where he too delivered a blazing endorsement.

Their roles did not stop there. Buttigieg appeared in a short video with Biden and encouraged his supporters to donate to the campaign. Klobuchar recorded a radio advertisement that was soon airing in Minnesota and directed her staff and supporters to help Biden win the state. O’Rourke taped a robocall that dialed up voters across Texas, blasted his supporters with an email he quickly wrote on the flight to Dallas and live-streamed himself taking Biden to Whataburger late Monday night.

At Whataburger, Biden had pulled on a T-shirt with O’Rourke’s face on it.

“Put me in coach,” Biden said. “I’m ready to play.”

Biden’s team could feel the momentum — but had no inkling of the victory he was about to see. They figured Biden would do well in the Southern states and Virginia and that he “had a shot in North Carolina.”

As soon as the polls closed in Virginia, Biden was declared the winner. Then came Alabama. Then North Carolina.

As Biden was declared the winner of Minnesota, his headquarters staff broke into a chant of “Amy! Amy! Amy!” After he won Texas, they jokingly renamed Whataburger “Whatabiden.”

At a victory party in Los Angeles, Biden warmly credited Klobuchar, O’Rourke and Buttigieg. But his rendition of Democratic unity stopped there as he took a few swipes at Sanders, without naming him.

“If you want a nominee who’s a Democrat, a lifelong Democrat, a proud Democrat, an Obama-Biden Democrat,” he said, “join us.”

On the other side of the country in Vermont, Sanders watched the results at home with his family, then traveled to a rally in nearby Essex Junction, where the mood was tense at times. The crowd roared in approval when CNN reported that Sanders won Vermont, Colorado and Utah, but it grew quiet when Biden’s victories were displayed.

Biden’s good run would continue. On Wednesday, Bloomberg would drop out and endorse Biden. On Thursday, Warren, too, would leave the race.

Biden was in a position unimaginable days earlier — the delegate leader and the only major candidate left to wage a one-on-one fight with Sanders for the nomination.

“It seemed to be spontaneous, almost. I don’t think it was a serious, planned thing. I just think people want to win,” said former Ohio governor Ted Strickland, a Biden ally who lambasted Sanders as too divisive. “No one wants his finger pointing at them for the next four years, so people got it together.”