A resurgent Joe Biden rode a wave of momentum to win Texas and eight other states, while Bernie Sanders was leading in Super Tuesday's biggest race in California.
In a surprisingly strong showing, Biden rolled to victories across the South, Midwest and New England on the biggest day of voting in the Democratic campaign. Americans in 14 states cast ballots to select a challenger to Republican President Donald Trump in the 3 November election.
In what would be the day's biggest upset, Biden was projected by Edison Research to have won Texas, the biggest prize after California. Sanders invested heavily in Texas and was counting on its sizeable Latino population to propel him to victory.
"For those who have been knocked down, counted out, left behind, this is your campaign," said Biden, the former vice president who had performed poorly in the first three nominating contests but broke through with a win in South Carolina.
"We are very much alive," he told roaring supporters in Los Angeles.
Sanders, the one-time front-runner who had hoped to take a big step toward the nomination on Tuesday, won Colorado, Utah and his home state of Vermont, Edison Research said.
Fox News and AP projected Sanders won California, whose 415 delegates represent the largest haul in the nominating contest. But Edison Research and other networks held off declaring a winner as results trickled in. Maine was a toss-up, with both Biden and Sanders winning roughly a third of the vote with 73 percent of the precincts reporting.
With overwhelming support from African-American, moderate and older voters, Biden also swept to wins in Alabama, Arkansas, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee and Virginia.
It was a spectacular turn for Biden, whose campaign was on life support after poor showings in Iowa and New Hampshire. Until a week ago he trailed Sanders in most state and national opinion polls.
But Biden's blowout win in South Carolina on Saturday provided a burst of new momentum, fueling a wave of endorsements from elected Democratic officials and former presidential rivals including Pete Buttigieg, a former mayor of South Bend, Indiana, and US Senator Amy Klobuchar of Minnesota.
Biden spent most of the past few months in the early voting states and did not personally campaign in several of the states he ultimately won, including Minnesota, in the months leading up to Super Tuesday.
Biden called Klobuchar Tuesday night to thank her for the Minnesota win, after she dropped her own presidential bid and publicly endorsed Biden at his rally in Texas the night before, a Biden campaign official said.
One third up for grabs
More than one-third of the delegates who will pick the eventual nominee at a July convention were up for grabs on Tuesday, which provided some clarity at last in a muddled race for the White House.
The results left Michael Bloomberg, the billionaire former New York mayor who spent more than half a billion dollars on advertising, largely out of the running, with his only victory coming in the US territory of American Samoa.
Bloomberg campaign officials said he would reassess whether to stay in the race on Wednesday, but they said that did not mean he would drop out.
The results were also disappointing for Senator Elizabeth Warren, who finished well behind Sanders and Biden in most states and trailed them in her home state Massachusetts.
Just a week ago, Biden was hoping to stay within reach of Sanders in delegates, giving him a chance to catch up as the race moved on. But initial tallies showed him leading Sanders in delegates for the day 268 to 194, with hundreds more to be allocated.
To date, Biden leads Sanders in delegates 321 to 254. A candidate needs to win 1991 delegates to win the Democratic nomination on the first ballot at the Democratic National Convention in July.
The campaign continues an accelerated pace through the next two weeks, when nearly another 1000 delegates are up for grabs, including the swing state of Florida on 17 March.
Biden also accomplished his main Super Tuesday goal of muscling aside Bloomberg and consolidating support from moderates to turn the race into a one-on-one contest against Sanders.
Without naming him, Sanders took direct aim at Biden during a rally with supporters in Vermont, criticizing his 2002 vote to authorize war in Iraq and his support for global trade deals that Sanders opposed.
"We're going to win the Democratic nomination and we are going to defeat the most dangerous president in the history of this country," Sanders said, referring to Trump.
Bad night for Warren, Bloomberg
Bloomberg was a wild card heading into the voting, as he joined the competition for the first time. He was winning 15 percent or more of the vote, enough to pick up some delegates, in Tennessee, Texas, Colorado, Utah, California and Arkansas.
The moderate skipped the first four contests and bombarded Super Tuesday and later voting states with ads, but saw his poll numbers slip after coming under fire during Democratic debates over past comments criticized as sexist and a police policy he employed as New York's mayor seen as racially discriminatory.
Biden is hoping to build a bridge between progressive Democrats' desire for big structural change and more moderate Democrats' yearning for a candidate who will be able to win over enough independents and Republicans to oust Trump.
That effort gained fresh momentum on the eve of Tuesday's voting as moderates Buttigieg and Klobuchar endorsed Biden after withdrawing from the race.
In Texas, which Biden unexpectedly won with 33 percent of the vote compared to Sanders' 30 percent, 56 percent of the voters said they favored a candidate who could beat Trump over someone who they agreed with on major issues. Among those voters, nearly 40 percent chose Biden, over Sanders who had 26 percent, according to Edison Research exit polls.
Biden pulled off his victories despite being vastly outspent on ads by Bloomberg, who had spent $US377 million on ads by 23 February, and Sanders, who spent $20m. Biden spent $4.7m during the same period, according to the Wesleyan Media Project, which tracks ad spending and content.
The pace of the Democratic race begins to accelerate after Super Tuesday, with 11 more states voting by the end of March. By then, nearly two-thirds of the delegates will have been allotted.
The next contests, on 10 March, will be in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington state.