The search for the Democrat challenger to Republican President Donald Trump in the November election narrowed on Wednesday to a choice between left wing Bernie Sanders and centrist Joe Biden, who staged a comeback in Super Tuesday voting.
Mr Biden was set to win 10 of the 14 states up for grabs on Tuesday, including delegate-rich Texas. The former vice president roared ahead in the overall tally of delegates who will choose a presidential nominee at the Democratic convention in July.
His strong performance ended Mr Sanders' status as the Democratic front-runner and prompted former New York Mayor Michael Bloomberg to drop out of the race.
Mr Bloomberg on Wednesday gave up his presidential campaign and endorsed Mr Biden, after spending more than $500 million of his own money on ads across the United States. But Mr Bloomberg failed to deliver convincing results on Tuesday, not winning any of the 14 states.
"I'm sorry we didn't win," Mr Bloomberg, 78, told a crowd of supporters in a New York City hotel. "A viable path to the nomination just no longer existed."
He said he was endorsing Mr Biden because was most likely to beat Mr Trump. "I hope you will join me in working to make him the next president."
His media company, Bloomberg News, lifted its election coverage restrictions hours after his announcement to quit the race.
His exit ended a novel electoral strategy, a vast experiment in political advertising, as Mr Bloomberg, 78, skipped the four early-voting states and instead focused on the 14-state Super Tuesday contest. His only victory on Tuesday came in the US territory of American Samoa.
The billionaire did not say whether he would spend part of his fortune to help Mr Biden, but being a centrist too, Mr Bloomberg's absence from the race will help the former vice president.
In a tweet to Mr Bloomberg, Mr Biden wrote: "I can’t thank you enough for your support — and for your tireless work on everything from gun safety reform to climate change. 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."
Mr Sanders, a senator and democratic socialist who is popular with young voters, lashed out at what he said was "the kind of venom we’re seeing from some in the corporate media," and attempted to draw a stark contrast between himself and Mr Biden.
"What this campaign, I think, is increasingly about is: Which side are you on?" Mr Sanders told a news conference in his state of Vermont.
He criticised Mr Biden's position on trade and for voting for the Iraq war.
"One of us in this race led the opposition to the war in Iraq – you're looking at him. Another candidate voted for the war in Iraq."
But Mr Sanders later admitted that he was "disappointed" by Tuesday evening's results and said he was falling short in inspiring young people to vote.
“Of course I’m disappointed,” told a news conference at his campaign office in Burlington, Vermont. “I would like to win every state by a landslide. It’s not going to happen.”
Speaking about the youth vote, he said: “Have we been as successful as I would hope in bringing young people in? And the answer is no.”
Speaking to reporters in Los Angeles, Mr Biden said he did not want the primary race to turn ugly.
"What we can't let happen in the next few weeks is let this campaign turn into a campaign of negative attacks," Mr Biden said. "The only thing that can do is help Donald Trump."
In another move that could reshape the race, Elizabeth Warren, 70, is "talking to her team to assess the path forward," a campaign aide said.
The liberal senator, who was seeking to become the nation's first female president, had disappointing results across the board on Tuesday, including coming in third in her home state of Massachusetts.
The Washington Post reported on Wednesday that top allies of Ms Warren and Mr Sanders are discussing ways for their camps to unite, with the expectation that Ms Warren will drop out the campaign soon.
A resurgent Mr Biden, 77, rolled to electoral victories across the South, Midwest and New England, and is now set go head-to-head with Mr Sanders, who won three states and led in California, which has the biggest pool of delegates. The results in the Western state won’t come in for several days or even weeks.
Mr Biden, whose campaign had been ailing just weeks ago, registered surprise victories in Texas and Massachusetts. But on the eve of Super Tuesday, a number of endorsements were made in Texas by former candidates Pete Buttigieg, Amy Klobuchar and Beto O'Rourke, who all pledged their support to Mr Biden.
Tallies after Tuesday showed Mr Biden leading Mr Sanders in delegates overall by 433-388. A candidate needs 1,991 delegates to win the Democratic nomination on the first ballot at the July convention.
US stocks surged on Wednesday as investors cheered the centrist’s strong night. Healthcare stocks provided the biggest boost as Mr Sanders and his "Medicare for All" proposal, which would eliminate private health insurance, looked less likely to come to fruition.
The Dow Jones Industrial Average and the S&P 500 both gained more than 4 per cent, rising for only the second time in 10 days after being battered by fears about the coronavirus outbreak.
Mr Biden argues that after two terms by President Barack Obama's side and decades as a US senator, he has the experience both to beat Mr Trump and run the country. He has promised greater access to healthcare and to mend relations with traditional US allies in Europe that were frayed by Mr Trump's "America First" foreign policy.
Mr Sanders, 78, wants to establish a free universal healthcare system, forgive student loan debt and enact the "Green New Deal" of sweeping economic policies to fight climate change.
If Ms Warren drops out, the Vermont senator might benefit from some of her supporters shifting to him.
In an email to her campaign staff, Warren’s campaign manager, Roger Lau, offered a sobering assessment of Super Tuesday, just three weeks after he said internal projections showed she would finish in the top two in eight states.
Warren, he said, would “take time right now to think through the right way to continue this fight.”
Mr Sanders said he had spoken to Warren over the phone on Wednesday.
"Elizabeth Warren is a very, very excellent senator, she has run a strong campaign. She'll make her own decision in her own time," Mr Sanders said.
Mr Trump said Mr Sanders would have done better on Tuesday if Warren had dropped out of the race beforehand and backed him. "Had Warren endorsed Bernie, we would have had a different story now,” he told reporters in the White House.
In Tuesday's biggest upset, Biden was projected by Edison Research to have won Texas, the largest prize after California. Sanders invested heavily in Texas and was counting on its Latino voters to propel him to victory.
Mr Sanders, who had hoped to take a big step on Tuesday toward the nomination, won Colorado, Utah and his home state of Vermont, Edison Research said.
Mr Biden, with overwhelming support from African-American, moderate and older voters, swept to wins in Alabama, Arkansas, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee and Virginia. Edison Research on Wednesday projected Biden to win Maine.
Fox News and the Associated Press projected Sanders winning California, where 415 delegates are up for grabs. By Wednesday afternoon, Sanders was ahead by 8.7 percentage points with almost 90 per cent of precincts reporting.
But a series of changes in the state that meant to boost voter turnout and smooth its primary election led to a surge in last-minute voters, computer problems and short-staffing that appeared to catch elections officials by surprise.
Long lines, sluggish computer connections and general confusion plagued polling places statewide — raising serious questions about the ability of the most populous state to handle November’s general election, when millions more voters are expected.