Media playback is unsupported on your device Media caption 'Little Michael' Bloomberg will hurt Biden - Trump

US President Donald Trump has taunted former New York City Mayor Michael Bloomberg, who has taken a key step towards joining the White House race.

Speaking to reporters early on Friday, Mr Trump said of the billionaire: "There is nobody I'd rather run against than little Michael."

Mr Bloomberg later filed paperwork for the Democratic presidential primary in Alabama.

But he has so far not announced that he is running for president.

What else did President Trump say?

On Friday, Mr Trump said Mr Bloomberg "doesn't have the magic" to make it to the White House.

He continued: "He's not going to do well, but I think he's going to hurt Biden actually."

Calling him "a nothing", Mr Trump said that Mr Bloomberg "will fail" if he joins the Democratic race.

Media playback is unsupported on your device Media caption Bloomberg to BBC in 2018: 'I'd like to make a difference'

What did Bloomberg's aide say?

In a statement late on Thursday, 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 presents 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.

Image copyright Getty Images Image caption Mr Bloomberg and Mr Trump, pictured here playing golf in 2007, were once friendly

Why is he running now?

Analysis - Anthony Zurcher, BBC News, Washington

Why is he contemplating a run for the highest political job in the land just a few months after announcing he would watch 2020 from the sidelines?

Here are a few theories.

Because he thinks he can win

Because he wants to shape the debate

Because he can afford to

The top one is the obvious response. Bloomberg has plenty of pollsters and political strategists at his disposal and is reported to be a very data-driven businessman. It doesn't take an advanced degree in quantitative analysis, however, to realise that the Democratic field, even at this (relatively) late date is still in flux.

There are four candidates at or near the top of early state and national primary polls - Joe Biden, Elizabeth Warren, Bernie Sanders and Pete Buttigieg. All of them have their strengths, of course, but all of them also have obvious weaknesses.

What happens after after Alabama?

Mr Bloomberg's advisers are reportedly preparing the 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 Warren welcomed 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".

Image copyright Chip Somodevilla/Getty Images Image caption Bernie Sanders, Joe Biden, and Elizabeth Warren are currently leading the field of Democratic candidates

Who is Bloomberg?

Mr Bloomberg's net worth is $52bn (£40bn), according to Forbes - 17 times more than 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.

Originally a Democrat, Mr Bloomberg became a Republican to mount his campaign for New York mayor. Now regarded as a moderate 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.

But he is more conservative on topics like the economy and policing.