Michael Bloomberg, the billionaire former mayor of New York City, has entered the Democratic presidential race.
The move adds another moderate voice to a crowded field of White House contenders.
The move is an about-face for Mr Bloomberg, 77, who said in March that he would not make a 2020 run for the White House.
"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 said in a statement on Twitter.
"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."
I'm running for president to defeat Donald Trump and rebuild America.— Mike Bloomberg (@MikeBloomberg) November 24, 2019
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
Mr Bloomberg's candidacy presents a late challenge to the pack of Democratic contenders near the top in opinion polls - fellow centrists Joe Biden, the former US vice president, and Pete Buttigieg, the mayor of South Bend, Indiana, and prominent liberal US Senators Elizabeth Warren and Bernie Sanders.
Mr Bloomberg has been critical of anti-Wall Street crusader Ms Warren and her plan to tax the super wealthy to pay for programmes ranging from universal healthcare to free college tuition.
He will compete with Mr Biden and Mr Buttigieg to become the moderate alternative to the liberal agendas of Ms Warren and Mr Sanders.
He has won allies in the party with his advocacy and philanthropy on climate change and in fighting gun violence, pouring millions into groups pushing for more restrictive gun laws. Mr Bloomberg served as mayor of New York, the largest US city, from 2002 to 2013.
Ranked by Forbes as the eighth-richest American with an estimated worth of $53.4 billion, Mr Bloomberg will have the advantage of being able to self-finance his campaign and pour millions of dollars into advertising and hiring staff.
He had filed paperwork on Thursday with the US Federal Election Commission to run for president. He already had filed paperwork to be eligible for the Democratic primary in Alabama and four other states with early deadlines for ballot qualification.
He announced earlier in November a $100 million online ad campaign targeting Mr Trump in four battleground states.
Mr Bloomberg will face significant disadvantages because of his late start, which means he will be playing catch-up with rivals who have been putting together campaign staffs for months ahead of the state-by-state battle for the Democratic nomination.
To counter that, he plans to pursue the unorthodox strategy of skipping the four states with early nominating contests in February, beginning on 3 February in Iowa, and focus primarily on the states that hold primaries and caucuses starting on so-called Super Tuesday on 3 March.
No winning presidential contender has ever pursued a similar strategy.
Mr Bloomberg already has come under fire from liberal critics who say he would be the wrong choice for a Democratic Party turning against corporate money in politics and dedicated to ending income inequality in America.
He also is certain to face renewed scrutiny over his record as New York's mayor and as the head of his own media company for nearly 40 years.
He apologised this month for New York's "stop and frisk" policy, that allowed police to stop and search people on the street, which was decried as racist for overwhelmingly targeting black men. Black voters are a critical Democratic voting bloc.
Mr Bloomberg also has been mocked by some critics for attempting to ban sodas sold in cups larger than 16 ounces (473ml), in an effort to reduce sugar consumption in the interest of public health amid high rates of obesity in the United States. The proposal was ultimately struck down by New York courts.
He will face questions about his decision to run for New York mayor in 2001 as a Republican. He switched to independent before a run for a third term in 2009. In 2018, while weighing whether to run for president, he switched his party registration again and became a Democrat.
At 77, he also will be the second-oldest candidate among the Democrats in a race in which age has been an issue as the party debates whether it is time for a new generation of leadership. Mr Sanders, who took time off from the campaign trail after a heart attack in October, is the oldest at 78. Mr Biden is 76. Ms Warren is 70. President Trump is 73.
Born 14 February 1942 into a Jewish family -- his father was an accountant at a dairy – Mr Bloomberg excelled in electrical engineering at Johns Hopkins University before earning a Harvard MBA.
He went to work at the Salomon Brothers investment bank in 1966 and rose quickly to the rank of partner before leaving with a $10 million bonus in cash and bonds.
He and partner Matthew Winkler founded financial news firm Bloomberg LP, revolutionising stock exchanges with its innovative computer terminals that financiers could access data far quicker than before.
Public opinion polls show most Democrats do not share Mr Bloomberg's dissatisfaction with the other Democratic contenders. A Monmouth University poll taken in late October and early November found three-fourths of Democrats were satisfied with their choice of candidates and just 16% wanted someone else.
Mr Bloomberg considered running in 2016 but decided against it, endorsing Democrat Hillary Clinton instead.
Late entrants into recent presidential races have not fared well. Wesley Clark, a retired US general, entered the Democratic presidential race in September 2003, and Fred Thompson, a Republican US senator, entered the Republican race in September 2007 after months of speculation.
Neither made much of a dent in the race before dropping out. Mr Clark won one state nominating contest, in Oklahoma, and Mr Thompson did not win any.