The 77-year-old businessman issued a statement on his campaign website under the slogan: “A new choice for Democrats, proven leadership for America.”
He said: “I’m running for president to defeat Donald Trump and rebuild America.
“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.
“The stakes could not be higher. We must win this election. And we must begin rebuilding America.
“I believe my unique set of experiences in business, government, and philanthropy will enable me to win and lead.
“As a candidate, I’ll rally a broad and diverse coalition of Americans to win. And as president, I have the skills to fix what is broken in our great nation. And there is a lot broken.”
His announcement ends weeks of speculation as to whether he would throw his hat into an already crowded ring of Democratic hopefuls.
Mr Bloomberg’s considerable fortune will give him staying power in the race although many Democrats have suggested his entrance is less than helpful, partly because there were already more than a dozen candidates, but also because his decision will be seen as reflecting a lack of confidence in other centrist candidates such as Joe Biden, Pete Buttigieg and Amy Klobuchar.
On Saturday, CNN, citing analysis from Kantar Media/CMAG, reported that Mr Bloomberg would launch his campaign with a $37m (£29m) television advertising campaign.
He is not the only late candidate, with former Massachusetts governor Deval Patrick having announced earlier this month that he too would be seeking the Democratic nomination.
Mr Bloomberg was formerly a Republican but became an independent during his second term as mayor of New York City, a position he held between 2002 and 2013.
He made his fortune after founding Bloomberg, a financial data and media company, in 1981.
The man he hopes to take on next November, fellow native New Yorker Donald Trump, began the trash talking even before Mr Bloomberg’s formal announcement.
Speaking last week, the president reacted to signals that Mr Bloomberg was poised to join the race by saying: “I know Michael. He became just a nothing. He was really a nothing.
“He’s not going to do well but I think he’s going to hurt Biden actually. But he doesn’t have the magic to do well. Little Michael will fail. He’ll spend a lot of money. He’s got some really big issues. He’s got some personal problems and he’s got a lot of other problems.
“But I know Michael Bloomberg fairly well – not too well, fairly well, well enough. He will not do very well. And if he did I’d be happy. There is nobody I’d rather run against than Little Michael, that I can tell you.”
The president may have been stung by news networks gleefully comparing their estimated wealth – according to Forbes, Mr Bloomberg is worth $53bn while Mr Trump can lay claim to a relatively paltry $3.1bn.
If they do face off it will be an unusual election between candidates who previously supported each other’s party – while Mr Bloomberg was once a Republican, Mr Trump has at times backed the Democrats.
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