Michael Bloomberg Photo: VCG

Billionaire media mogul Michael Bloomberg, the former mayor of the largest US city, jumped into the race for the Democratic US presidential nomination on Sunday as a moderate with deep pockets unabashedly aiming to beat fellow New Yorker Donald Trump in the November 2020 election.

Bloomberg's belated entry into the race - just three months before the first of the state-by-state party nominating contests - reflects his skepticism that any of the other 17 Democratic candidates can unseat the Republican president.

"I'm running for president to defeat Donald Trump and rebuild America," Bloomberg, a 77-year-old former Republican, said in a statement launching his campaign. 

"We cannot afford four more years of President Trump's reckless and unethical actions." 

The move represents an about-face for Bloomberg, who said in March that he would not run for president. He will compete with former vice president Joe Biden and Mayor Pete Buttigieg of South Bend, Indiana to become the moderate alternative to liberal US Senators Elizabeth Warren and Bernie Sanders.

Bloomberg, founder and CEO of prominent media company Bloomberg LP and a leading philanthropist, has a financial advantage over his Democratic rivals. It already is on display as he has spent at least $31 million in television ads that will run in states across the country over the next two weeks, a campaign spokesman said.

He has won allies in the party with his advocacy and philanthropy on climate change and in fighting gun violence, pouring millions of dollars into groups pushing for more restrictive gun laws.

Ranked by Forbes as the eighth-­richest American with an estimated worth of $53.4 billion, Bloomberg joins activist Tom Steyer as the second billionaire to enter the Democratic race and will have the advantage of being able to self-finance his campaign and pour millions of dollars into advertising and hiring staff.

Bloomberg will face questions about his decision to run for New York mayor in 2001 as a Republican. He switched to independent before a run for a third term in 2009. In 2018, while weighing whether to run for president, he switched his party registration again and became a Democrat.