Former New York mayor Mike Bloomberg filed federal papers Thursday declaring himself a Democratic candidate for president, a potentially disruptive move that could upend the party’s nomination fight this spring.

The filing, coming just eight months after Bloomberg ruled out a bid because he believed it would be too hard to win the Democratic nomination, reflects his view that the field of Democratic contenders was not well positioned to win next year and that a candidate with his experience, political moderation and deep pockets would have a better chance of defeating President Trump in a general election.

Advisers said Thursday that the filing was a step toward running for president, following several state ballot registrations, but not an official announcement or public signal that he had made a final decision. An adviser said the timing of the filing was triggered by his earlier application for a spot on the Alabama ballot.

But Bloomberg’s team has been moving swiftly over the past two weeks to rebuild a presidential campaign operation that was scuttled after he decided earlier this year not to run.

One of the world’s richest men, with a net worth estimated by Forbes to be $53 billion, Bloomberg is positioned to be a force to counter the candidates who emerge from the first four nominating contests in Iowa, New Hampshire, Nevada and South Carolina. In defiance of the traditional nominating calendar, Bloomberg has planned to skip those contests to spend heavily in states that will vote in March — including California, Texas and 12 other states that will award delegates on Super Tuesday.

As the other candidates focus on the early states, Bloomberg has looked at building his organization in more delegate-rich states while also spending $115 million or more on ads and voter registration efforts targeting Trump in states such as Wisconsin, Michigan and Pennsylvania that are likely to be crucial in the general election.

His bid would be built around his effectively bottomless pockets, since no presidential campaign has ever come close to spending the resources at Bloomberg’s disposal. His advisers expect to work without a preset budget or any outside fundraising efforts, a fact that will bar him from participation in the December party debates, which require candidates to meet a donor threshold.

“This is going to be a unique campaign, with a real focus on grass-roots organizing making sure people are on the ground engaging with voters and engaging with citizens, at the same time investing in paid media and an unprecedented digital strategy,” said Bloomberg’s likely campaign co-chair, Stephen Benjamin, who is the mayor of Columbia, S.C., and has been working with the team planning a potential campaign.

Bloomberg’s longtime advertising strategist, Bill Knapp, has signed on to work on the campaign. Mitch Stewart, a senior field organizer for both of Barack Obama’s presidential campaigns, has been tapped to plan ground operations, marking an early decision to build an operation that reaches beyond television and digital advertising.

Stewart suggested that the strategy will build on relationships Bloomberg has already formed through his philanthropic efforts with mayors, environmentalists and gun-regulation activists, among others.

“Mike can uniquely build a robust infrastructure across the country, fueled by activists who have worked on issues with Mike for literally decades,” Stewart said.

Bloomberg, 77, is the financial backer of one of the largest standing grass-roots political efforts in the country to push for more gun regulation. His groups — Moms Demand Action, Students Demand Action and the Everytown Survivor Network — boast hundreds of local chapters with representation in every state. The groups made more than 100,000 phone calls and knocked on tens of thousands of doors this fall in Virginia to help elect Democrats in the state legislature, according to advisers.

“Our volunteers will come out and support him,” John Feinblatt, who oversees Bloomberg’s gun efforts, said when asked about a possible campaign. “As he travels around the country, you can expect them to show up.”

Bloomberg will also draw on extensive work funding mayoral efforts around the country. Benjamin, who was a prominent backer of Hillary Clinton’s campaign in 2016, is an alumnus of a training program for mayors that Bloomberg funds with Harvard University.

Another likely co-chair of the campaign, Louisville Mayor Greg Fischer, has won competitive grants for his city from Bloomberg Philanthropies.

“I can’t tell you how many times I hear about people who want to vote for a Democratic nominee but they are afraid of a Democratic nominee being too far to the left,” Fischer said. “So his name comes up.” Both have endorsed Bloomberg’s presidential ambitions.

But Bloomberg’s campaign will face fierce early head winds from a party electorate that has shown skepticism in recent years toward self-funding candidates and those with close Wall Street ties. Born in Medford, Mass., to a family of modest means, Bloomberg made his money selling information technology to bankers, traders and others in the finance industry.

For much of his political career, he was a Republican, registering as an independent in 2007 and rejoining the Democratic Party in October 2018. As mayor of New York, he endorsed President George W. Bush’s reelection in a speech at the 2004 Republican National Convention in New York. In 2016, he spoke at the Democratic nominating convention with a speech warning of a Trump presidency.

He has a mixed history of handling sexual harassment cases at his company that could prove a liability in the #MeToo era and has long been a critic of public labor unions, a key Democratic constituency, when they refuse to renegotiate their pension plans or to weaken workplace protections for teachers in public schools.

Bloomberg has been critical of Democratic proposals for a Medicare-for-all system that would effectively end private insurance. He has publicly lobbied against Trump’s trade war with China, where his company does business, as a job-killing mistake. He has supported increases in income-tax rates for the wealthiest Americans but has dismissed as probably unconstitutional the proposal by Sen. Elizabeth Warren (D-Mass.) for a wealth tax on the assets of billionaires such as himself.

“We need a healthy economy, and we shouldn’t be embarrassed about our system,” he said on a February visit to New Hampshire. “If you want to look at a system that’s noncapitalistic, just take a look at what was perhaps the wealthiest country in the world, and today, people are starving to death — it’s called Venezuela.”

Bloomberg attempted to address his greatest vulnerability with the key Democratic demographic of black voters Sunday, when he apologized for the mayoral policy that allowed New York police to stop and frisk people simply for appearing suspicious. The policy disproportionately targeted black and Latino residents.

“Today, I want you to know that I realize back then, I was wrong,” Bloomberg said at a predominantly black megachurch in Brooklyn.

In recent years, Bloomberg has been one of the biggest donors to federal elections, to liberal groups such as Planned Parenthood and to an effort to close coal-fired power plants with the Sierra Club. He has also been a major backer of smoking-cessation efforts around the world. He will become the second Democratic billionaire in the race; another heavyweight donor, Tom Steyer, with a net worth estimated at less than $2 billion, joined the contest in July.

In 2016, Bloomberg pollster Doug Schoen produced projections that showed the former mayor with a clear shot at winning an electoral-college majority in a three-way contest against Sen. Bernie Sanders (I-Vt.) and Trump, largely by winning pluralities in swing states such as Florida, Michigan, Wisconsin and North Carolina. Schoen found a three-way contest with Trump and Clinton more difficult.

Ultimately, Bloomberg opted not to run for president on a third-party ticket in 2016, because he said he feared he would risk Trump’s election. In March, Bloomberg ruled out a separate campaign for the Democratic nomination because he said he was “clear-eyed about the difficulty of winning the Democratic nomination in such a crowded field.”

But Bloomberg has long boasted of having a historically high tolerance for political risk. In an interview in November 2018, he recalled fondly the steep odds he faced when he was considering his first run for New York mayor, as a Republican.

“I said, ‘Can I win?’ ” Bloomberg recalled of a conversation he had with his pollster Schoen. “And he said, ‘No, no chance.’ ”

Schoen predicted Bloomberg would get only a third of the vote. But Bloomberg ran anyway, spending $74 million on a campaign that concluded shortly after the 9/11 attacks. He won with 50.3 percent of the vote.