CHATTANOOGA, Tenn. — The 12th richest person in the world decided to bet more than $500 million late last year on the long-shot notion that Democrats would defy history and fail to elevate a front-runner after Iowa and New Hampshire's nominating contests.

Three months later, Mike Bloomberg’s jackpot is about to come due, as the former New York mayor, who has yet to appear on a ballot, emerges from the chaos of early February as a legitimate contender for the Democratic presidential nomination.

Bloomberg has benefited from a cascade of good luck — a botched and inconclusive Iowa caucuses process, the collapse of the former vice president Joe Biden’s polling dominance and the Democratic Party chairman’s decision to provide the mayor a path to the next debate. His potential to reshape the race has led his Democratic opponents, as well as President Trump, to begin focusing on him in earnest, even though he is not contesting Nevada and South Carolina, the next two states to vote.

“You have a billionaire literally trying to buy an election, and that’s not the politics we believe in,” said Ari Rabin-Havt, deputy campaign manager for Sen. Bernie Sanders (I-Vt.). Sanders called Bloomberg’s self-funded candidacy “precisely what the corruption of the American political system is all about” during a Saturday stop in Dover, N.H.

Sen. Elizabeth Warren (D-Mass.) echoed that sentiment in a discussion Sunday with reporters on her campaign bus in Rochester, N.H. “Are we going to be an America where you’ve got to be a billionaire or suck up to billionaires to become the party’s nominee?” she asked.

With the exception of Sanders, who finished strong in the Iowa caucuses and won Tuesday’s New Hampshire primary, Bloomberg’s rivals are heading toward the biggest voting day of the primary — Super Tuesday on March 3 — with no clear momentum. Biden and Warren have been hobbled with two embarrassing losses. Former South Bend, Ind., mayor Pete Buttigieg and Sen. Amy Klobuchar (D-Minn.), who finished second and third, respectively, in New Hampshire, face the challenge of winning over nonwhite voters who have not yet embraced their campaigns.

Bloomberg, who has been dominating the airwaves unchallenged in many states, said Wednesday that he does not want to engage directly with his rivals at the moment. He is not, however, above more oblique references.

“We don’t need a revolution. We want evolution,” he told a mostly white crowd of hundreds at a rally in Tennessee on Wednesday, a nod to Sanders. “And we need a nominee who can deliver it.”

Yet it became clear in recent days that the scrutiny is coming from all directions.

As New Hampshire voters went to the polls Tuesday, the Biden campaign joined with Trump in trying to spread around a 2015 audio clip in which Bloomberg defended the targeted frisking of black and Latino New Yorkers during his time as mayor, a policy he has since renounced and apologized for promoting.

“Ninety-five percent of your murders, murderers and murder victims fit one M.O. You can just take the description, Xerox it and pass it out to all the cops,” he was recorded saying at an Aspen Institute event.

The quotes caught fire after they were posted online Monday by a liberal podcast host, though the Republican opposition research firm America Rising claimed Tuesday that it had previously flagged the clip and that there were others like it to come. Biden’s director of African American engagement, Trey Baker, emailed the audio around to allies on Monday night.

“Feel free to share with your networks,” Baker wrote in the email, which was obtained by The Washington Post. Symone Sanders, an adviser to Biden, went on Fox News to call the clip “sad” and “despicable.” “Mayor Bloomberg is unvetted,” she later warned.

Trump retweeted and then deleted the footage on his account Tuesday morning, and his campaign manager, Brad Parscale, subsequently posted another clip of Bloomberg defending “stop and frisk” with the hashtag #Bloombergisaracist. As president, Trump has been a vocal defender of stop-and-frisk policing, even announcing in 2018 that he wanted the Justice Department to encourage the practice in cities.

“I don’t think those words reflect how I led the most diverse city in the nation,” Bloomberg said in a brief news conference Wednesday. “And I apologized for the practice and the pain that it caused.”

His campaign also announced the endorsements of three more black lawmakers, Reps. Lucy McBath (D-Ga.) and Gregory W. Meeks (D-N.Y.), and Del. Stacey Plaskett (D-Virgin Islands).

Bloomberg’s advisers expect more incoming over the coming weeks as his profile rises. Rivals have previously taken issue with his past positions on financial regulatory reform, the allegations of sexual harassment at his eponymous media company and his strong support for charter schools.

“It became crystal clear today that we are the opponent that Trump least wants to face,” Bloomberg adviser Howard Wolfson said in response to the Trump tweet Tuesday. “The entire apparatus has engaged against us, which is a sign that we are doing well.”

Rival campaigns have also begun to raise concerns about whether Bloomberg’s strength could help speed Sanders’s path to the nomination by further splitting the moderate vote in the party. Under Democratic rules, candidates can only accrue delegates if they clear 15 percent support thresholds, which are measured on both statewide and congressional district levels.

Two national polls this week, by Monmouth University and Quinnipiac University, found that Sanders was the only candidate comfortably clearing that bar, raising the possibility that he would win a disproportionate number of delegates if his rivals remain bunched up below 10 percent.

“Right now, it looks like he is undermining his stated goal for getting in the race, which is preventing Bernie Sanders from being the nominee,” said a senior strategist for a rival Democratic campaign, who spoke on the condition of anonymity to discuss strategy. “Just as we do see moderates starting to surge, whether it is Pete or Amy, he is stepping in to stop the momentum.”

Bloomberg’s aides, who privately share the same concern about the moderate vote splitting in a way that helps Sanders, said the burden is on other candidates “to get out” if they have failed to perform in the early contests. The aides spoke on the condition of anonymity to discuss strategy.

“Everybody has got to make a decision,” Bloomberg said Wednesday about the dilemma presented by a crowded field. “You have to balance between what you think is right for the country/party, to get a candidate to beat Donald Trump, and on the other hand your personal aspirations.”

Behind the scenes, Bloomberg, who is worth more than $50 billion, has stepped up his debate preparations, which began last year, as he faces what is likely to be the next major test of his campaign. He needs one more party-approved national poll over a 10 percent threshold before Tuesday to qualify for the Feb. 19 debate in Las Vegas.

The mock debates with advisers have focused less on mastering policy details, with which Bloomberg is largely conversant, than on coaching him to connect on a more human level with the consequences of policy, advisers say.

The Bloomberg strategy for winning the nomination depends heavily on him earning the support of nonwhite voters in the March primary states, where most of the party’s delegates will be awarded. Those voters consistently voiced support for Biden in polling last year, and Biden’s aides have expressed confidence in recent weeks that they will be able to maintain the support, despite the New Hampshire and Iowa setbacks.

“We have been focused from the outset on appealing to a very diverse electorate,” Biden adviser Kate Bedingfield said. “Our strength with African American voters and Latino voters has been very durable. And it is very hard to buy that support.”

Democratic strategists unaffiliated with the current campaigns say the next big question for Bloomberg is whether his growing support, which has been buffeted by a massive investment in television advertising, will convert into a base of voters who retain real enthusiasm for his campaign. Polls continue to show Bloomberg trailing other Democratic contenders in favorability.

“He needed to draw an inside straight, and so far he has,” Barack Obama’s former campaign manager Jim Messina said. “The question is: When it is snowing in a Midwestern state on March 3, are voters going to get up and wade through the snow to vote for Mike Bloomberg?”

The campaign has moved aggressively to backstop the possibility that the television advertising only provides a sugar high of support.

Dan Kanninen, Bloomberg’s states director, announced Monday that the campaign head count now exceeds 2,100, including at least 40 paid staff in 18 states and a team of 400 working out of Times Square headquarters. While other campaigns traveled to New England the past weekend, his field team staged 1,200 events in more than 30 states, with 600,000 logged voter conversations, advisers said.

“We are not just running television ads or digital ads,” Kanninen said. “We are on the ground with staff, volunteers, leaders, endorsers, community members, and we are engaging with voters.”

In broadcast and digital advertising, Bloomberg has already spent more than $344 million, according to Advertising Analytics, a tracking firm.

Bloomberg argued that he needs to spend the money because he got in the race so late. “I’m not trying to buy the election,” he said.

That advertising includes a range of different approaches, from conventional spots attacking Trump on health care and boasting of Bloomberg’s working relationship with Obama to viral social media attempts to attack Trump in a space where he is often dominant.

One recent social media spot redubbed a Trump address from the White House to make it look like he was just repeating the words “lie” and “unfair” over an electronic beat. The campaign superimposed graphics on the video of a dancing gingerbread cookie with its legs on fire. The 2 minute and 30 second video has been viewed more than 2.1 million times on Twitter.

Almost to a person, attendees at his rally in Chattanooga said they were familiar with his paid advertising.

“I love his ads,” said Annette Allen, a voter from Signal Mountain, Tenn., who said her only priority was finding a Democratic candidate to beat Trump. “My vote is for sale.”

Sean Sullivan and Annie Linskey contributed to this report.