A total of 17 Democratic candidates are vying to take on President Trump.
Former Vice-President Joe Biden, Massachusetts Senator Elizabeth Warren and Vermont Senator Bernie Sanders are the frontrunners.
Who is Michael Bloomberg?
Mr Bloomberg’s net worth is $52bn (£40bn), according to Forbes. This is nearly 17 times more than that of Mr Trump ($3.1bn).
Mr Bloomberg was a Wall Street banker who went on to found the financial publishing empire that bears his name.A philanthropist, he has donated millions of dollars to educational, medical and other causes.
Originally a Democrat, he became a Republican to mount a successful campaign for mayor of New York City in 2001.
He went on to serve three terms as mayor until 2012, rejoining the Democratic Party only last year.
Mr Bloomberg is a large financial backer of gun-control advocacy group Everytown for Gun Safety, which he helped found in 2014.