US media tycoon Michael Bloomberg quit the Democratic presidential race and endorsed frontrunner Joe Biden Wednesday after being snubbed by voters on Super Tuesday despite blowing more than half a billion dollars on his campaign.

"Three months ago, I entered the race for president to defeat Donald Trump. Today, I am leaving the race for the same reason: to defeat Donald Trump -- because it is clear to me that staying in would make achieving that goal more difficult," he said in a statement.

The billionaire former mayor of New York spent more than $500 million on his presidential run, much of it on a huge advertising campaign, but failed to win any of the 14 states on offer on Super Tuesday -- the most important day in the Democratic primary season.

"Mini Mike Bloomberg just 'quit' the race for President," Trump tweeted.

"I could have told him long ago that he didn't have what it takes, and he would have saved himself a billion dollars, the real cost. Now he will pour money into Sleepy Joe's campaign, hoping to save face. It won't work!"

The 78-year-old Bloomberg had bet his run on Super Tuesday after deciding to skip the first four voting states but all he picked up was a consolation victory in American Samoa, a tiny territory in the Pacific.

CBS estimated that the former mayor of New York, who entered the race in November pitching himself as the only candidate who could defeat Trump, spent around $18 million per delegate that he won.

Democratic presidential candidate former New York City mayor Mike Bloomberg spent an estimated $18 million for each delegate he won
Democratic presidential candidate former New York City mayor Mike Bloomberg spent an estimated $18 million for each delegate he won GETTY IMAGES NORTH AMERICA / JOE RAEDLE

His adverts were still running on major television networks after his announcement that he was dropping out and backing Biden, who emerged as the frontrunner with a string on huge victories Tuesday.

"Defeating Trump starts with uniting behind the candidate with the best shot to do it. It's clear that is my friend and a great American, @JoeBiden," Bloomberg said.

"I've known Joe for a very long time. I know his decency, his honesty, and his commitment to the issues that are so important to our country - including gun safety, health care, climate change, and good jobs," he added.

Bloomberg said he had been "immensely proud" of his campaign but it had become clear that staying in the race would make achieving the goal of defeating Trump more difficult.

Bloomberg, who was regularly attacked by Trump as "Mini Mike," during his short-lived campaign, touted his commitment to fighting climate change and gun control.

But he struggled to shake off controversy surrounding his "stop and frisk" policy when he was New York mayor which indiscriminately targeted minorities.

He also endured a widely criticized debate performance in Nevada two weeks ago.