Former New York mayor Michael Bloomberg is making plans to enter the Democratic presidential primary campaign this week, a reflection of anxiety among party elites about the unsettled field of current contenders.

Bloomberg, who as one of the world’s richest men would bring significant financial resources to his own campaign but also inflame the populist wing of the party, plans to file paperwork and has dispatched staff to Alabama to ensure he can get onto the ballot in a state that has a Friday filing deadline. He has been calling top party officials to let them know of his plans and could make an announcement as early as next week.

The move marks a major reversal for Bloomberg, who announced in March that he would not run for president, and also serves as a public rebuke of the performance so far of former vice president Joe Biden, who has attempted to build a coalition of the same moderate Democrats that Bloomberg would court.

One of the driving reasons Bloomberg decided against joining the race earlier this year — he announced his decision seven weeks before Biden entered — was his view that Biden was too formidable a contender. But in the months since, Biden has been underwhelming, remaining among the race’s leaders but halting in his debate performances and stumbling over raising the tens of millions necessary to mount a strong campaign.

Bloomberg adviser Howard Wolfson would not take on Biden personally in confirming the billionaire’s plans but did allude to questions about the field.

“We now need to finish the job and ensure that Trump is defeated,” Wolfson said. “But Mike is increasingly concerned that the current field of candidates is not well positioned to do that.”

Concern is also rippling through the Democratic Party over the others at the top of the candidate pile. The liberal policy positions advanced by Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.) are seen by many as unpalatable in general election contests in the states expected to determine the winner. Sanders’s health is also an issue after his October heart attack. South Bend, Ind., Mayor Pete Buttigieg has attracted attention and raised substantial money, but he has little support among African Americans, one of the party’s most important constituencies.

Bloomberg, 77, has been outspoken in his opposition to Warren’s and Sanders’s intentions to raise taxes on the extremely wealthy like himself, and on Thursday they returned the ill sentiments.

“The billionaire class is scared and they should be scared,” Sanders wrote on Twitter after news of Bloomberg’s possible entry became public.

“Welcome to the race, @MikeBloomberg!” Warren tweeted, providing a link to the impacts her policies would have on billionaires. She also sent out a fundraising email saying “the wealthy and well connected are scared.”

It is still possible that Bloomberg would not ultimately enter the race, but he is taking steps to ensure he will be on the ballot. Alabama’s deadline is Friday, and New Hampshire’s is Nov. 15.

The Democratic field has winnowed recently from two dozen to 16, but Bloomberg’s decision could also open the door to other announcements. Former attorney general Eric H. Holder Jr. has not ruled out a possible entry, nor has the party’s 2016 nominee, Hillary Clinton. Oprah Winfrey, an early backer of Barack Obama, has repeatedly begged Disney chief executive Bob Iger to jump into the race, but he has so far been unwilling.

“His getting in the race is certainly going to stimulate thought and provocations that weren’t there before,” former senatorHarry M. Reid (D-Nev.) said of Bloomberg, whom he spoke to on Thursday. He is scheduled to talk with Holder early next week.

“Bloomberg doesn’t do things halfway,” Reid added. “He’s going to stir up some conversation.”

Bloomberg has decided not to raise money for his bid if he does move forward, which would preclude him from entry in the Democratic debates under rules that require a growing number of donors to qualify. “He has never raised a dime for his campaigns, and he is not about to start,” said a person familiar with his thinking, who spoke on the condition of anonymity to speak frankly about the planning.

It is also possible that he decides to skip the first four voting states, an unorthodox strategy that could upend the regular nominating process and place far more emphasis on the Super Tuesday contests of March 3, when the race will become more nationalized — and more expensive. His advisers have said in the past that they did not plan to operate with a preset budget if he mounted a presidential effort.

News of Bloomberg’s potential entry was first reported by the New York Times.

Bloomberg’s adviser Wolfson, in confirming his plans, highlighted his history of building a business from scratch and becoming a “high-impact philanthropist.” He has also spent lavishly on politics, donating more than $100 million in the 2018 midterms and playing a role in Tuesday’s off-year elections, in which Democrats romped in the suburbs that often yearn for a moderate standard-bearer.

Bloomberg was open earlier this year about his desire to get into the Democratic primary race. He made trips to Iowa and New Hampshire, testing the waters for a campaign before deciding against it, and instead planned to fund an independent political operation that would work against Trump.

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

Bloomberg is the second billionaire Democrat to reverse an earlier decision. Tom Steyer had initially decided against running but recently joined the field.

A Monmouth University poll in March found that Bloomberg was disliked by just about as many Democrats as liked him — with 27 percent viewing him favorably, 26 percent unfavorably, and nearly half saying they had no opinion or hadn’t heard of him.

Bloomberg’s entrance would reignite a battle over how much the richest Americans should be taxed. In January, on one of his final New Hampshire trips before deciding against a bid, Bloomberg told The Washington Post that Warren’s wealth tax was “probably unconstitutional,” and he warned that seriously pursuing it could wreck the country’s prosperity.

“We need a healthy economy, and we shouldn’t be embarrassed about our system,” Bloomberg said. “If you want to look at a system that’s not capitalistic, just take a look at was perhaps the wealthiest country in the world, and today people are starving to death. It’s called Venezuela.”

Warren, who has eschewed big-money fundraisers, had warned from the outset of her campaign that a self-funding billionaire was a threat to the party.

“We ought to be building grass-roots support,” Warren told reporters after her very first campaign event of 2019, in Council Bluffs, Iowa. “We ought to be building a movement. And the way we do that is with lots of involvement from lots of people. Not having billionaires buy these campaigns, whether we’re talking about super PACs or self-funding.”

Bloomberg’s entrance would give her and Sanders another foil but could pose challenges not only to Biden and Buttigieg but a host of moderates who have struggled to gain traction, including Montana Gov. Steve Bullock and Sens. Michael F. Bennet (Colo.) and Amy Klobuchar (Minn.)

“I still believe Biden is best positioned to defeat Trump, but Bloomberg obviously fears that Warren or Sanders might prevail in the primary and then end up losing to Trump,” said Jon Cooper, a Democratic fundraiser and prominent Biden supporter. “For those more centrist Democrats who share that concern, Bloomberg’s entry into the race could offer a belt-and-suspenders approach.”

When he decided not to run earlier this year, Bloomberg’s advisers had pointed to the apparent strength of other moderate potential candidates, most notably Biden. But he has grown increasingly concerned, particularly as impeachment proceedings have threatened to ensnare Biden over his actions regarding Ukraine.

“As incredibly concerned as Mike was about Trump remaining president, after Ukraine he become ever more concerned. He is convinced the president is an existential threat,” said the person familiar with Bloomberg’s thinking.

“We have seen in data that Democrats are extremely focused on nominating the best candidate to defeat Donald Trump. There is a supreme desire to have the best possible candidates to run in November. We think we can make a very, very strong case that that is Mike Bloomberg,” the person said.

David Weigel and Michelle Ye Hee Lee contributed to this report.