endorsed former Vice President Joseph R. Biden Jr. on Wednesday, concluding that his own path to the Democratic nomination had evaporated after he spent hundreds of millions of dollars and won no states on Super Tuesday.
After courting moderate voters across the map and briefly rising to a prominent place in national polls, Mr. Bloomberg’s candidacy unraveled in just a few weeks: His numbers plunged after a debilitating clash with Senator Elizabeth Warren in a debate last month, and much of his support appeared to flee to Mr. Biden over the past few days.
Mr. Bloomberg, who was elected mayor of New York City as a Republican before becoming one of the Democratic Party’s biggest financial backers, is said to be weighing his plans to spend heavily for Mr. Biden and other Democrats in the 2020 general election.
According to a person briefed on his deliberations, Mr. Bloomberg reached the decision to withdraw from the race on Wednesday morning, after meeting with his inner circle of advisers at the Upper East Side townhouse that is the headquarters of his political and philanthropic empire — several miles away from the Times Square campaign office where in just a few months he assembled an enormous team of aides in a late-starting bid for the presidency.
But the likelihood that he would exit the race was already apparent on Tuesday night, as election returns showed Mr. Bloomberg falling into third place or worse in states he had once hoped to win outright. He carried only one contest, in American Samoa.
Mr. Bloomberg’s aides told political allies late on Tuesday that he would reassess his candidacy in the morning, and acknowledged that the picture emerging from the March 3 primaries was not an encouraging one. They began informing some of his top supporters of his intention on Wednesday morning.
Mr. Bloomberg and Mr. Biden spoke by phone on Wednesday morning, a person briefed on the call said. Shortly after Mr. Bloomberg announced his exit, Mr. Biden posted an appreciative tweet saying he would be counting on Mr. Bloomberg’s help to defeat President Trump.
While Mr. Bloomberg’s departure from the race was meant to help unite the moderate wing of the Democratic Party behind Mr. Biden, it also had the potential to enrage supporters of SenatorBernie Sanders, who have long regarded Mr. Bloomberg as a malignant force in the 2020 campaign.
As a candidate, Mr. Bloomberg had argued against the danger of not just the re-election of Mr. Trump but also the nomination of Mr. Sanders, whom Mr. Bloomberg criticized as an unelectable candidate with unrealistic or bad ideas. Mr. Sanders in turn branded Mr. Bloomberg as an avatar of the oligarch class that his campaign is trying to bring to heel.
Mr. Bloomberg’s endorsement of Mr. Biden is unsurprising in most respects: The two moderates have enjoyed a cordial relationship over the years, and when Mr. Bloomberg initially passed on a presidential campaign last year, his advisers said it was because he had judged Mr. Biden as too strong for Mr. Bloomberg to overcome.
But Mr. Bloomberg reversed course last fall, after watching Mr. Biden’s public struggles in the race and conducting private polling that convinced him that there was a new opening for him. His decision to enter the primary stunned and angered Mr. Biden’s campaign, and at times over the past few months their rivalry has grown sharply personal.
Yet Mr. Biden may be disinclined to hold a grudge: Mr. Bloomberg has vowed to mount a heavily funded outside-spending effort for the eventual Democratic nominee, and his endorsement Wednesday made plain that he sees Mr. Biden as the worthiest recipient of that largess.
Mr. Bloomberg’s candidacy was unprecedented in its financial firepower, amounting to a no-expenses-spared effort to take control of a presidential race. His Democratic rivals accused him of seeking to buy the presidency, and Mr. Bloomberg often came close to embracing that idea: In his speeches, he frequently made allusions to his vast personal fortune and presented himself to Democratic voters as the candidate with “the record and the resources” to win the general election.