After Mr. Biden’s big wins on Super Tuesday Mr. Bloomberg endorsed him and said that he planned to remain involved in the 2020 presidential campaign

Michael R. Bloomberg dropped out of the presidential race and backed Joseph R. Biden Jr. on Wednesday, throwing the financial might of the Democratic Party’s biggest benefactor behind the former vice president’s campaign as Senator Bernie Sanders of Vermont vowed to wage a long battle for the nomination.

Mr. Sanders, regarded a week ago as having a clear upper hand over Mr. Biden, sounded chagrined after losses in states from Maine to Texas where he was hoping for huge turnout. There was a surge in voter participation, but it lifted Mr. Biden instead.

Mr. Sanders now faces pressure to show he can expand his political base, and he acknowledged that he had not yet managed to transform the electorate with a wave of young people.

The sudden shift in political momentum has redefined the Democratic race at breakneck speed. Since Saturday night, when Mr. Biden won South Carolina in a landslide, much of the Democratic establishment has aligned behind him. Two rivals, Pete Buttigieg and Senator Amy Klobuchar, dropped out and endorsed Mr. Biden on Sunday and Monday; he won 10 states on Tuesday, including Texas and North Carolina; and Mr. Bloomberg backed him on Wednesday.

Only Senator Elizabeth Warren of Massachusetts remains as a major candidate. Her campaign said on Wednesday that she was assessing her path forward in view of another night of dispiriting election returns, including a third-place finish in her home state, that left Ms. Warren without a single victory after a month of primaries and caucuses.

Mr. Sanders said on Wednesday that he had spoken with Ms. Warren, who for months had been his most formidable rival from the Democrats’ populist wing. But Mr. Sanders said he applied no public pressure on her to stand down and shared no knowledge of her intentions, if he had any.

Some supporters have urged Ms. Warren to take her time, while others are pressing her to capitalize on her existing leverage by dropping out and endorsing Mr. Biden, according to Democrats who have spoken to her. But Ms. Warren made it clear to one supporter on Wednesday that she was not going to act hastily and that it may be at least another day before she makes up her mind.

In a further boon to Mr. Biden, Mr. Bloomberg signaled in his concession speech that he intended to keep wielding his multibillion-dollar fortune against President Trump. The former New York City mayor had previously pledged to keep spending large sums of money to help Democrats in the general election, even if Mr. Bloomberg did not become the nominee. (The Sanders campaign has said it would not welcome that kind of help.)

“I am cleareyed about our overriding objective, and that is victory in November,” Mr. Bloomberg said, adding, “I will not be our party’s nominee, but I will not walk away from the most important political fight of my life.”

Advisers to Mr. Bloomberg said on Wednesday that they had not yet settled on a detailed spending plan for the general election, but Mr. Bloomberg is said to be keenly interested not only in the presidential race but also in Democratic efforts to take full control of Congress. Members of his enormous campaign staff were told that it would take perhaps a week for final decisions about how Mr. Bloomberg might reorient his campaign machinery.

Mr. Biden has not faced a sustained challenge in weeks, having been largely written off by his rivals after his embarrassing fifth-place finish in New Hampshire. It remains to be seen whether Mr. Biden will be able to fully extend the energy that has propelled his campaign over the last few days, and how deftly he might be able to grapple with a determined opponent like Mr. Sanders.

But Mr. Sanders acknowledged that he was disappointed by the results from contests this week in 15 states and territories, two-thirds of which Mr. Biden won. And in a striking and uncharacteristic admission, Mr. Sanders conceded that his campaign had not managed to generate the soaring turnout among young people that he had been counting on to secure the nomination.

“Have we been as successful as I would hope, in bringing young people in? And the answer is no,” Mr. Sanders said. “We’re making some progress.”

Campaigning in West Beverly Hills, Calif., on Wednesday, Mr. Biden brandished the breadth and diversity of his support to reject Mr. Sanders’s accusations that he is an instrument of the political establishment.

“The establishment are all those hard-working, middle-class people, those African-Americans, those single women,” Mr. Biden said, referring to voters who turned out in force for him this week.

Both Biden and Sanders supporters will be looking for victories next Tuesday in primaries and caucuses in Michigan, Missouri and four other states. Mr. Bloomberg’s exit, and his immediate move to back Mr. Biden, had the potential to anger Sanders supporters, who have long regarded Mr. Bloomberg as a malignant force in the 2020 campaign, and stoke resentment among progressives that party power brokers were again taking aggressive steps to thwart Mr. Sanders.

But Mr. Bloomberg’s decision to leave the race came primarily from an unsparing assessment of his own feeble prospects.

Mr. Biden’s campaign raced to harness his new momentum and Mr. Sanders sought to refocus his rattled political operation for a newly grueling fight. The Biden campaign has told allies that it is seeking to rapidly hire campaign staff to strengthen its presence in states where the former vice president has built little or no organized infrastructure because of his financial difficulties so far.

Mr. Sanders’s team is hoping that his more developed campaign apparatus might give him a chance to slow Mr. Biden’s rise in the half-dozen states that vote next week, most significantly in the large swing state of Michigan. Addressing reporters in Burlington, Vt., on Wednesday afternoon, Mr. Sanders described the Michigan primary as “enormously important” and said he had high hopes to win.

Repeating a litany of criticism he leveled at Mr. Biden in his election-night speech Tuesday, Mr. Sanders indicated he would focus over the next week on attacking Mr. Biden’s record of supporting what he described as “disastrous trade agreements” that had been particularly damaging to Midwestern states like Michigan. Mr. Sanders said several times that “Joe is going to have to explain” various other parts of his record, including his support for the 2003 invasion of Iraq and the 2008 bailout of the financial industry.