"I believe I would defeat Donald Trump in a general election," Bloomberg wrote in March. "But I am clear-eyed about the difficulty of winning the Democratic nomination in such a crowded field."

"He's tired of being the almost-ran," said one former Bloomberg adviser on Thursday after The New York Post reported news of his moves. "Mortality is weighing heavily on him. This is the last time that he can run."

Bloomberg will almost certainly not qualify for the next debate in Atlanta on Nov. 20, and he will likely have to spend tens of millions of dollars in the next few weeks to make the debate stage in December, potentially setting a record for campaign spending in a short amount of time. His current wealth is estimated at $52 billion, per Forbes.

Bloomberg and his money have the potential to scramble the Democratic field, where no clear frontrunner has emerged. He is most similar ideologically with candidates like Biden and South Bend Mayor Pete Buttigieg and could threaten their candidacies or divide their potential voters.

Asked why Bloomberg changed his mind and whether he was underwhelmed by Biden's performance on the trail, an aide said that "it's not about any one candidate." But the aide added: "He is worried about the state of the Democratic primary campaign and the possibility that we could lose in November. He wants to avoid that more than anything else."

Bloomberg's money also could further sharpen the existing divisions in the field between the left-wing, populist candidates and more moderate, pragmatic ones. While advisers to Biden and Buttigieg declined to comment on Bloomberg's entry, Warren's team and Sanders himself reacted with near glee at the potential contrast he would offer.

"More billionaires seeking more political power surely isn't the change American needs," said Sanders campaign manager Faiz Shakir. Sanders weighed in on Twitter, saying "[t]he billionaire class is scared and they should be scared."

Warren welcomed Bloomberg to the race on Twitter and linked to a page about her wealth tax, adding: "If you're looking for policy plans that will make a huge difference for working people and which are very popular, start here."

David Axelrod, Democratic strategist and longtime aide to Barack Obama, said concerns about Biden clearly contributed to Bloomberg's change of heart.

“There’s no question that Bloomberg’s calculus was that Biden was occupying a space, and the fact that he’s getting in is a clear indication that he’s not convinced Biden has the wherewithal to carry that torch," Axelrod said. "So yeah, I don’t think this is a positive development for Joe Biden."

Bloomberg has poured hundreds of millions of dollars into Democratic causes in recent years, earning him goodwill among many progressive activists and especially those connected with climate change and gun control. His funding of the gun control advocacy group Moms Demand Action has given the left a powerful counter to the National Rifle Association. But Bloomberg, who ran as a Republican in his first mayoral bid in 2001 before becoming an independent and then a Democrat, also faces vulnerabilities on the left on criminal justice and the regulation of Wall Street.

Last year, he defended the stop-and-frisk policing tactics during his mayoralty as helping decrease the murder rate without violating civil rights. "I think people, the voters, want low crime,” Bloomberg told The New York Times. “They don’t want kids to kill each other.”

He has also repeatedly criticized Warren's wealth tax proposal and suggested her policies toward the rich could lead the United States on a path to becoming Venezuela.

Bloomberg would be the second billionaire to make a late entry in the contest, after investor Tom Steyer jumped in over the summer. Steyer spent tens of millions in advertising to get on the debate stage but has failed to rise from the bottom of polls.

Longtime Democratic strategist Joe Trippi said it's possible Bloomberg could follow a similar trajectory. But Trippi said that he didn't think the former New York City mayor would enter the race without a realistic path to victory.

"I don’t think Wolfson or those folks would do this if they haven’t polled," Trippi said. "It’s not like they woke up and decided to do this. I’m sure they have data.”

Michael Nutter, the former Philadelphia mayor who wanted Bloomberg to run and then endorsed Biden, echoed that sentiment Thursday evening. “He doesn’t make rash decisions. This is serious," Nutter said. "Bloomberg has his own serious record of achievement in politics, philanthropy and certainly business. It’s kind of a triple threat.”

Bloomberg's deep pockets may allow him to chart an unconventional path to the nomination that is less dependent on the early states of Iowa and New Hampshire. Candidates often rely on making a splash in those contests to gain momentum and raise more money, but Bloomberg will likely be able to stay in the race as long as he wants without such pressure.

“It’s tough in Iowa," said Matt Paul, who ran Hillary Clinton's caucus operation in the state in 2016. "It’s tough with 90 days away. If he does this, this is not going to be an Iowa-centered campaign. If he does this, this is going to be a very unorthodox campaign."

Steve Benjamin, the mayor of Columbia, South Carolina, sat next to Bloomberg at an event in Washington last month and came away with an inkling of what might come.

“I sensed that he was still very interested in the race,” Benjamin said. “I gave him the lay of the land in South Carolina ... He wanted to hear more about it.”

Sally Goldenberg contributed to this report.