NEW YORK, United States (AFP) — Billionaire Michael Bloomberg formally entered the race to be US president yesterday, pledging to “rebuild America” as he joined the crowded field of Democrats seeking to take on Donald Trump.

Bloomberg’s personal fortune of US$50 billion is likely to shake up the open Democratic contest, with the declaration of his candidacy coming just three months before the first primary votes.

“The stakes could not be higher. We must win this election. And we must begin rebuilding America,” Bloomberg, 77, said on his website as a US$30-million ad campaign touting his candidacy hit US airwaves.

The announcement ended weeks of speculation that the former New York mayor was preparing for a run at the White House and highlights the fluid nature of the Democratic race ahead of the first votes in Iowa in February.

Bloomberg, the co-founder and CEO of the media company that shares his name, becomes the 18th candidate vying to be the Democratic nominee to take on Trump in 2020.

His entry comes at a time when candidates are usually dropping out. Ten Democrats have already ended their bids.

Bloomberg had prepared the groundwork for a possible run at the Democratic nomination in recent weeks by registering as a candidate in primary voting states and filing paperwork with the Federal Election Commission.

“We cannot afford four more years of President Trump’s reckless and unethical actions,” the billionaire businessman said.

“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,” Bloomberg added.

Former vice-president Joe Biden leads the race ahead of left-wingers Elizabeth Warren and Bernie Sanders, with moderate Pete Buttigieg in fourth place, according to national polls.

Bloomberg is poised to run as a centrist and analysts suspect that he could take away some of the support enjoyed by fellow moderate Biden.

Some believe that his “self-made man” image and support for fighting global warming makes him the best challenger for Trump.

Bloomberg’s advisors have suggested that he may not campaign in the four states that will hold primaries in February, including Iowa. Instead he is expected to focus on the 15 primaries scheduled for March 3, known as “Super Tuesday”, including the most populous states of California and Texas.