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******** Media captionBloomberg to BBC in 2018: ‘I’d like to make a difference’
What did Bloomberg’s aide say?
In a statement, Bloomberg adviser Howard Wolfson said: “We now need to finish the job and ensure that Trump is defeated.
“But Mike is increasingly concerned that the current field of candidates is not well positioned to do that.”
Mr Bloomberg is said to be fully aware such a belated entry to the race could present challenges in states like Iowa and New Hampshire, where other Democratic contenders have been campaigning for months.
The Bloomberg team reportedly sees a possible pathway through the so-called Super Tuesday contests in March, when 14 states, including California, Alabama and Colorado, will vote on a single day for their preferred White House nominee.
Mr Bloomberg, 77, considered running for the White House as an independent candidate in 2008 and 2016.
In March of this year he said he would not join the 2020 race.
What happens next?
If Mr Bloomberg does get on the ballot in Alabama by Friday, he will still have to register in other states.
His advisers are reportedly preparing the necessary paperwork for other states with upcoming deadlines. Both Arkansas and New Hampshire require candidates to file by next week.
State-by-state votes, known as primaries and caucuses, will be held from February next year to pick a Democratic White House nominee.
The eventual winner will be crowned at the party convention in Wisconsin in July. He or she is expected to face President Trump, a Republican, in the general election in November.
A total of 17 Democratic candidates are vying to be the party’s standard-bearer.
Joe Biden, Massachusetts Senator Elizabeth Warren and Vermont Senator Bernie Sanders are the current front-runners.
What’s the other reaction?
Mr Biden told media on Friday that he had “no problem” with Mr Bloomberg joining the Democratic field.
“Michael is a solid guy,” Mr Biden said. “Let’s see where it goes.”
Ms Warrenwelcomed Mr Bloomberg to the race on Twitter, linking to her own campaign website and suggesting the former mayor take a look for potential policy plans.
In tweet seemingly directed at Mr Bloomberg,Mr Sanders wrote: “The billionaire class is scared and they should be scared.”
Some recent opinion polls have suggested that Ms Warren and Mr Sanders – who are more politically liberal than Mr Biden – might face an uphill battle against Mr Trump.
The Republican National Committee said in a statement that the billionaire’s prospective entry “underscores the weak Democrat field”.
Who is Bloomberg?
Mr Bloomberg’s net worth is $ (BN) £ 40 bn), according to Forbes. This is nearly (times more than that of Mr Trump ($ 3.1bn).
He was a Wall Street banker before going on to create the financial publishing empire that bears his name.
He staged a successful campaign for New York mayor in 2001, remaining in office for three consecutive terms through 2013.
A philanthropist, he has donated millions of dollars to educational, medical and other causes.
What does Bloomberg believe in?
Originally a Democrat, Mr Bloomberg became a Republican to mount his campaign for New York mayor in 2001.
Now regarded as a Medium Democrat, he rejoined the party only last year.
Mr Bloomberg has liberal views on issues such as climate change, gun control, immigration and abortion rights.
He was credited this week with helping Democrats win control of Virginia’s legislature, after his gun-control advocacy group Everytown for Gun Safety injected $ 2.5m into the state’s election.
But Mr Bloomberg is more conservative on topics like the economy and policing.
As mayor, he defended NYPD’s stop-and-frisk policy, which critics say disproportionately targeted African Americans and Hispanics. Black voters are a vital constituency for Democrats.
At city hall, Mr Bloomberg banned supersize sodas to prevent obesity, but was overruled by the state’s Supreme Court.