Billionaire media mogul Michael Bloomberg, the former mayor of New York, entered the race for the Democratic US presidential nomination on Sunday.
The moderate former Republican is aiming to beat fellow New Yorker Donald Trump in the November 2020 election.
Mr Bloomberg's late entry into the race, three months before the first of the state-by-state party nominating contests, reflects his scepticism that any of the other 17 Democratic candidates could unseat the Republican president.
"I'm running for president to defeat Donald Trump and rebuild America," Mr Bloomberg, 77, said while launching his campaign.
"We cannot afford four more years of President Trump's reckless and unethical actions."
Mr Bloomberg said in March that he would not run for president.
He will compete against former vice president Joe Biden and Mayor Pete Buttigieg of South Bend Indiana to become a moderate alternative to liberal senators Elizabeth Warren and Bernie Sanders.
Mr Bloomberg, founder and chief executive of prominent media company Bloomberg and a leading philanthropist, has a financial advantage over his Democratic rivals.
He has spent at least $31 million (Dh113.8m) in television ads that will run in states across the country over the next two weeks, a campaign spokesman said.
Mr Bloomberg has won allies in the party with his advocacy and philanthropy on climate change and pouring millions of dollars into groups pushing for more restrictive gun laws.
He will be playing catch-up with rivals who have been putting together campaign staffs for months.
Ranked by Forbes as the eighth-richest American, with an estimated worth of $53.4 billion, Mr Bloomberg joins activist Tom Steyer as the second billionaire to enter the Democratic race.
He has the advantage of being able to finance his own campaign and pour millions of dollars into advertising and hiring staff.
Mr Bloomberg announced in November a $100m online ad campaign against Mr Trump in four battleground states.
"We do not believe that billionaires have the right to buy elections," Mr Sanders said in a Twitter post on Sunday.
"That is why multi-billionaires like Michael Bloomberg are not going to get very far in this election."
While some Democratic candidates had warned against making the election all about Mr Trump, Mr Bloomberg kept the focus squarely on the incumbent president, with whom he has been well acquainted for decades.
Mr Trump was a notable New York real estate developer during Mr Bloomberg's three terms as mayor from 2002 to 2013.
"I know what it takes to beat Trump, because I already have. And I will do it again," he said.
Mr Bloomberg stressed his success as a self-made businessman from humble roots, a "doer and a problem solver, not a talker".