Mike Bloomberg, the former New York mayor who poured half a billion dollars into his presidential campaign, dropped out of the race on Wednesday and endorsed the surging candidacy of Joe Biden, whose string of victories on Super Tuesday has upended the nominating contest.

Sen. Bernie Sanders (I-Vt.), who lost 10 of 14 state primaries to Biden on Tuesday, adopted a more aggressive tone with the former vice president Wednesday as Sen. Elizabeth Warren (D-Mass.) flew home to Boston to reassess her campaign, weighing whether to end her bid and allow liberals in the party to unite behind Sanders the way moderates quickly coalesced for Biden.

After a head-spinning four days, a primary race that began with a historically large and diverse field — powered by a half-dozen women attempting to tap into an activated female electorate — has now boiled down to two white men in their late 70s who each have spent about a half-century running for political office.

Biden and Sanders are now preparing to catapult their candidacies into a new round of contests over the next two Tuesdays, when 10 more states will vote and award nearly 900 additional delegates, a stretch that could determine the race.

As the results from the states that voted on Super Tuesday became more definitive, the shape of the campaign shifted swiftly and forcefully. Sanders cruised into Super Tuesday hoping to surge to a potentially insurmountable lead. But now, Biden is narrowly ahead — and looking at a more favorable map as demographics in the upcoming contests largely tilt in his direction. Biden has 433 delegates to Sanders’s 388, with California still tallying results.

Voter turnout in several states was dramatically higher than in 2016, with Democratic voters motivated to choose a nominee who they hope can unseat President Trump. But Sanders, whose campaign has long argued that it was expanding the electorate with new, younger voters, conceded that had not happened.

“Have we been as successful as I would hope in bringing young people in?” Sanders said to reporters at a campaign office in Burlington, Vt. “The answer is no.”

Sanders for months has had trouble taking a sharp line against Biden, jabbing occasionally at him as if they were in a fight on the Senate floor rather than a brawl for the Democratic nomination. But the future of his candidacy may depend on whether he can trigger a seismic shift in how voters view the former vice president.

“Joe Biden is someone I’ve known for many years. I like Joe, I think he is a very decent human being,” Sanders told reporters Wednesday. “Joe and I, we have a very different voting record. Joe and I have a very different vision for the future of this country, and Joe and I are running very different campaigns, and my hope is, in the coming months, we’ll be able to debate and discuss the very significant differences that we have.”

Sanders, who has been criticized for the bullying and vitriol that some of his supporters employ on social media, also reiterated that he does not want the campaign to turn into a “Trump-type effort where we’re attacking each other, where it’s personal attacks — that’s the last thing this country wants.”

Sanders and Warren spoke earlier on Wednesday, and she told him that she is assessing her campaign’s future.

Roger Lau, Warren’s campaign manager, sent an all-staff email Wednesday morning thanking them and explaining that the candidate will make a decision about her path forward in coming days.

“Last night, we fell well short of viability goals and projections, and we are disappointed in the results,” Lau wrote, according to a copy of the email obtained by The Washington Post. “We are going to announce shortly that Elizabeth is talking to the team to assess the path forward.”

He also asked that Warren be given some time to figure out her next move. “This decision is in her hands, and it’s important that she has the time and space to consider what comes next,” Lau wrote.

Warren and Sanders spoke by phone Wednesday. Their top surrogates and allies have discussed ways for them to unite and push a common liberal agenda, The Post reported Wednesday.

Aside from Warren, Rep. Tulsi Gabbard (D-Hawaii), who has earned a single delegate from American Samoa, is the only other candidate remaining.

Bloomberg's strange ride

Bloomberg’s exit from the race concluded one of the most unusual campaigns in politics, in which a billionaire tested the limits of money’s impact on the first stages of a highly volatile presidential primary race.

But after dismal results — picking up delegates but failing to win a single state outright — the former mayor on Wednesday morning called Biden to discuss his decision.

“I’m sorry we didn’t win. It’s still the best day of my life, and tomorrow’s going to be even better,” Bloomberg said to cheers from a crowd later in the day in New York. He reiterated that he “entered the race for president to defeat Donald Trump, and today I am leaving the race for the same reason.”

Bloomberg has pledged to employ large field staffs in six swing states in the general election, even though he is no longer a candidate. The billionaire’s data operation, Hawkfish, will also continue operating to elect Democrats up and down the ballot.

“Mike fully intends to put his resources and commitment in the broadest way possible behind Joe Biden’s candidacy,” said Tim O’Brien, a senior adviser to Bloomberg. “We have long-term leases and long-term contracts with the team, and the intention was always to put this big machine we have built behind whoever the nominee is.”

Bloomberg’s aides, however, have not announced whether they will take out ads in the primary campaign to help Biden or whether Bloomberg campaign staffers in upcoming primary states will work for Biden’s nomination.

Campaign finance rules generally give self-funded campaigns significant leeway to spend money, even if the funds support other candidates. Bloomberg also has the option of renaming or rehiring his staff as part of a separate independent group to help Biden, according to campaign finance experts.

Biden is not the only Democrat in the party eager for Bloomberg to stay invested.

Senate Minority Leader Charles E. Schumer (D-N.Y.), who benefited in 2018 from a $20 million donation Bloomberg gave to an outside group supporting Senate Democrats, released a statement after Bloomberg left the race.

“Mayor Michael Bloomberg’s deep and impactful record of action on gun safety, fighting climate change, and advocating for immigration reform made him a strong and worthy competitor in this primary,” Schumer said. “His continuing commitment to these fights — and to defeating the divisive and damaging Trump presidency — is statesmanlike and will permanently inure to his credit.”

There are six states voting on Tuesday, awarding 352 delegates. The largest prize is Michigan, but Missouri and Washington state also have large hauls. Biden is poised to do well in Mississippi, which has the same demographics as the Southern states he swept on Tuesday.

Perhaps the only major moment to change the race comes during a March 15 debate in Phoenix. Two days later, four important bellwether states — Arizona, Florida, Illinois and Ohio — vote and award 577 delegates, the second-biggest single-day haul remaining in the race.

By the end of March, nearly two-thirds of the delegates will have been awarded, and pressure will grow on any candidate who doesn’t have a chance at reaching a majority to drop out of the race. It takes 1,991 delegates to win the nomination.

Biden's Texas upset

Biden carried 10 of 14 Super Tuesday states. He swept the South and won in the Upper Midwest, carried Massachusetts and Maine, and scored a major upset by winning Texas.

Sanders won his home state of Vermont, as well as Colorado and Utah, and was expected to win the biggest delegate prize of California.

Most tallies had Biden with an overall lead of about 45 delegates. Perhaps more important, he was able to build a powerful coalition that included African American voters and the white working class, winning over women in the suburbs and disaffected Republicans.

Turnout was up sharply in many of the states, with nearly every one voting in bigger numbers than in 2016 and some states exceeding the 2008 contest that has been the modern barometer for voter enthusiasm.

In Texas, turnout was up about 60 percent compared with 2016, while Tennessee saw a 37 percent increase.

About 1.3 million voters in Virginia cast ballots in the Democratic primary, up from the previous record of about 986,000 during the 2008 contest.

There are still significant risks for Biden, and he will test whether voters care about his verbal gaffes or whether Republican attempts to re-litigate the actions of his son Hunter will gain additional traction. Biden’s son served on the board of Burisma, a Ukrainian gas company, while he was vice president and attempting to crack down on corruption in Ukraine. While there has been no evidence of wrongdoing, the actions of the Bidens became a part of Trump’s impeachment trial.

Biden’s campaign — and his growing list of prominent endorsers — has increasingly tried to showcase his character and empathy.

They say that making the contrast between a longtime politician who even opponents say is a good and decent person will provide a strong contrast with Trump, who has stretched traditional limits of civility and discourse.

“We set high expectations for the campaign [on Tuesday], and he absolutely blew through those expectations and had a record-breaking night,” said Rep. Cedric L. Richmond (D-La.), co-chairman of the Biden campaign. “People know Joe Biden. He’s authentic. He has empathy. He’s concerned, and he has experience.”

Annie Linskey, Michael Scherer and Sean Sullivan contributed to this report.