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 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' 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 Donald 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 by phone Wednesday. Their top surrogates and allies have discussed ways for them to unite and push a common liberal agenda.

Roger Lau, Warren's campaign manager, sent an all-staff e-mail Wednesday morning thanking them and explaining that the candidate will make a decision about her future 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 e-mail obtained by the Washington Post.

He 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.

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

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 black 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.

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.

"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 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."