MANCHESTER, N.H. — Former New York City Mayor Michael Bloomberg officially entered the 2020 race Sunday, ending several weeks of will-he-or-won’t-he speculation about a late entry into the already-crowded Democratic primary.

"I’m running for president to defeat Donald Trump and rebuild America," Bloomberg said on his campaign website. "We cannot afford four more years of President Trump’s reckless and unethical actions. He represents an existential threat to our country and our values. If he wins another term in office, we may never recover from the damage."

Bloomberg’s entry was preceded by news of a massive television ad buy — $31 million, according to Advertising Analytics, which said it was the single largest single week expenditure the firm had ever tracked. A $30 million buy in the final weeks of the 2012 race for then-President Barack Obama held the previous record.

The ad promotes Bloomberg's record as mayor in the aftermath of 9/11 as well as his post-mayoral work campaigning for gun control and against the coal lobby.

I’m running for president to defeat Donald Trump and rebuild America.



I believe my unique set of experiences in business, government, and philanthropy will enable me to win and lead.



Join our team: https://t.co/7ezlUeouqH pic.twitter.com/IyOeS3aWaF — Mike Bloomberg (@MikeBloomberg) November 24, 2019

It’s Bloomberg’s deep pockets and willingness to spend that could help him make up the difference of getting in several months after most of the already-established Democratic field. But his strategy to win is a risky one: skipping the early four nominating contests and instead running what longtime Bloomberg aide Howard Wolfson called a "broad-based, national campaign."

He’ll also come up against a field stacked with strong competition, some with similar messaging to his own — like former Vice President Joe Biden, who has also hinged his candidacy on his ability to beat President Donald Trump next November — and progressive Senators Elizabeth Warren and Bernie Sanders, who are running on platforms of more structural change. And they’ve all been running for months, building organizing machinery as they go.

Bloomberg also said that he wouldn't accept campaign contributions, which would prevent him from qualifying for the Democratic debates under the current rules. The choice to eschew donors has already drawn swift backlash from some of his opponents.

"This is an outrage, it's disgusting that somebody who thinks that this is the way that you win a presidency," Sanders' campaign manager Faiz Shakir said on Sunday, comparing Bloomberg's approach to Trump's 2016 campaign. "You had somebody who proclaimed that ‘I'm so rich that I can't be bought.’ And then he's proven when coming into office, that I fight for the people who I know in my elite bubble."

Warren — who hasn’t held back her feelings on billionaires, or Bloomberg — said Saturday night “this election should not be for sale,” later adding that she doesn’t think the race “is going to be about TV ads versus TV ads” but instead about “grassroots movements.”

Some voters in New Hampshire also expressed skepticism ahead of Bloomberg's announcement.

"He's a billionaire from New York just entering to muddy everything up or to protect him and his rich buddies," Elizabeth Nelson said on Saturday.

"I think it's too late," said Jenn Mapstone. "He missed his chance, there are so many people in front of him."

But Bloomberg Communications Director Jason Schecter said it’s not too late for the former mayor, citing polls that show Democratic voters have yet to firmly make up their minds on which candidates to support. Schecter said Bloomberg "has the skills to fix what is broken" and was motivated to run by concerns about "the possibility that we could lose next November" to Trump.

"We can’t afford another four years of this," he said.

Sunday's spot lays out Bloomberg's vision for the country, "where the struggling middle class will get their fair share." In what appeared to be a subtle challenge to the Medicare For All wing of the 2020 field, the ad proposes that "everyone without health insurance can get it and everyone who likes theirs can go ahead and keep it."

A longtime Democrat, Bloomberg ran for mayor in 2001 as a Republican before registered as an independent in 2007. He re-registered as a Democrat last year, prompting speculation that he was considering a presidential bid.

Bloomberg declined to enter the race last March. At the time, sources close to him told NBC News that they didn’t see a path to victory with Biden in the race. But consternation from certain Democratic circles about the strength of the field — Biden has lagged while Warren surged throughout the summer and early fall — reignited talk of a Bloomberg run.

Michael Bloomberg speaks at the Christian Cultural Center in Brooklyn on Nov. 17, 2019. Yana Paskova / Getty Images

Perhaps the clearest signal that he had decided to run was Bloomberg’s recent disavowal of the stop-and-frisk policy he implemented as mayor and fiercely defended for years. Speaking at a black megachurch in Brooklyn, New York, last weekend, Bloomberg said: "I got something really important wrong … I want you to know that I realized back then, I was wrong — and I’m sorry."

A key South Carolina politician — Columbia Mayor Steve Benjamin — was in the crowd that day and told NBC News a few days later that he was "moved" by the humility in Bloomberg’s apology. He said he planned to endorse him if the former mayor decided to officially run.

Other 2020 hopefuls who have been running for months, however, have reacted forcefully against Bloomberg’s foray into the field, even before Sunday’s official announcement.

"I don’t think a person, just because they have billions of dollars, should sit back and say, 'you know what, yeah, I think I’ll run for election right now and drop $100 million,'" Sen. Cory Booker, D-N.J., told a crowd in Concord, New Hampshire, on Saturday.

Sen. Amy Klobuchar, D-Minn., also in New Hampshire on Saturday, said she doesn’t "believe you get the best candidate when there's such a bias in terms of money. I don't believe that's how this works."

Klobuchar, who has joked that she raised $17,000 from her ex-boyfriends during her first Senate run, joked Saturday of Bloomberg’s record ad buy: “Man, I’m not going to be able to make that up with the ex-boyfriends.”