Joe Biden, counted out just days ago, took control of the race for the Democratic presidential nomination on Wednesday after racking up stunning primary victories over Bernie Sanders and earning the endorsement of deep-pocketed billionaire Michael Bloomberg.

President Donald Trump hailed the "incredible comeback" of the 77-year-old former vice president in the Super Tuesday contests and directed a stream of insults at Bloomberg after the fellow New Yorker ended his campaign.

"Those of you who have been knocked down, those of you who have been counted out - this is your campaign," Biden said in Los Angeles. "We had a great night last night and tremendous support from all across the nation."

Sanders, 78, the leftist senator from Vermont, expressed confidence in his chances of winning the nomination despite being "disappointed" with his showing on Tuesday, when the centrist Biden won at least 10 of the 14 states at stake.

"We go forward basically neck and neck," Sanders told reporters in his hometown of Burlington, Vermont, where the self-described democratic socialist got his start in politics as mayor four decades ago.

"I have every reason to believe that we're going to win this thing," Sanders said. "I firmly believe that we are the campaign to defeat Donald Trump."

In an interview with MSNBC's Rachel Maddow, Sanders warned it would not be easy. "Do not underestimate Donald Trump. He'll have endless amounts of money because he doesn't believe in the rule of law. He will combine federal agencies with his own campaign. He has a loyal group of supporters, so it's not going to be so easy to beat him."

Meanwhile, Sanders said he had spoken with fellow progressive candidate Elizabeth Warren, and confirmed that the senator from Massachusetts was "assessing" her campaign following her poor performance on Super Tuesday, when she failed to win a single state including her own.

"She will make her own decision in her own time," he said.

The Washington Post reported that the Sanders and Warren camps were talking about uniting, should she withdraw from the campaign in the near future.

Biden, who is making his third White House bid after failed runs in 1988 and 2008, welcomed Bloomberg's backing in the contest for the spot on the Democratic ticket against the Republican incumbent in November.

"This race is bigger than candidates and bigger than politics. It's about defeating Donald Trump, and with your help, we're gonna do it," he tweeted.

- 'Incredible comeback' -

The Democratic candidates will now turn their attention to the next primaries on March 10 in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington state.

Trump provided his commentary on the Democratic race to reporters at the White House and signaled how he will attack the new frontrunner. "It was a great comeback for Joe Biden, an incredible comeback when you think about it," Trump said.

Biden is running as a moderate, in contrast to the leftist Sanders, but Trump insisted that some of the former vice president's "handlers are further left than Bernie."

Trump noted that decisions by two centrists -- Pete Buttigieg and Amy Klobuchar -- to drop out and endorse Biden had electrified his previously struggling campaign.

Trump lambasted the weak performance by Bloomberg, who spent hundreds of millions of dollars of his own fortune on advertising.

"He's going to try and save face by putting some money into Biden's campaign," Trump said. "I don't think that's going to have an impact.

"He's doing that because he's spiteful," Trump added of Bloomberg. "He made a fool out of himself."

Biden's campaign had been on life support after the first three state nominating contests but was revived by a landslide victory in South Carolina on Saturday. He carried that momentum over into Super Tuesday, the biggest day of the election calendar so far.

Biden notched up victories in Virginia, Texas, North Carolina, Alabama, Oklahoma, Tennessee, Arkansas, Massachusetts, Maine and even Minnesota -- a state where Sanders had been expected to win handily.

Sanders won his home state Vermont, Colorado and Utah and looked to be headed for victory in California, the biggest prize of the night, as vote counting continued on Wednesday.

- 'Uniting' -

Bloomberg, 78, said that while he was leaving the race, he remained committed to beating Trump.

"I've always believed that defeating Donald Trump starts with uniting behind the candidate with the best shot to do it," Bloomberg said. 

"After yesterday's vote, it is clear that candidate is my friend and a great American, Joe Biden."

A key takeaway from Biden's long list of wins was his strong support among women and African Americans -- a vital piece in any Democratic presidential candidate's coalition.

Biden seemed to fare less well with the large Hispanic electorate, which in California reportedly went heavily for Sanders. But a victory in Texas, which also has a diverse population, suggests Biden has the capacity to build a broad coalition.

Many in the Democratic Party establishment have been desperate to stop Sanders, claiming he would be destroyed in an election where Trump would brand him a radical socialist.

Most national polls have both Biden and Sanders several points ahead of Trump in a general election matchup.