Former US vice president Joe Biden was projected to win at least nine large states on Super Tuesday but his main rival Bernie Sanders was seen picking up four, including California, the biggest prize of the night.
Mr Biden's White House hopes were suddenly revived with early projected wins in major southern battlegrounds Virginia, North Carolina and Alabama.
He went on to score wins in Tennessee, Arkansas, Minnesota, Oklahoma, and rival Elizabeth Warren's state of Massachusetts, while US networks projected a win in Texas, handing him a key victory over Mr Sanders.
"They haven't buried me yet," the 77-year-old told ecstatic supporters in Los Angeles earlier.
"I'm here to report, we are very much alive!"
Despite being heavily outspent by Mr Sanders and with only patchwork organisation on the ground in some states, Mr Biden claimed what he cheekily called the "#Joementum" in a tweet.
#Joementum pic.twitter.com/3UhSdLUT4Y
— Joe Biden (Text Join to 30330) (@JoeBiden) March 4, 2020
With partial results in from all 14 Super Tuesday states and one US territory, Mr Biden was forecast to win at least 175 delegates, with Mr Sanders getting at least 151, Michael Bloomberg eight, Elizabeth Warren five and Tulsi Gabbard one.
According to Edison Research exit polling, African Americans, older people and college graduates largely supported Mr Biden in the 14 nominating contests.
Latinos, millennials and white men largely backed Mr Sanders, helping him to likely wins in the western states of Colorado and delegate-rich California.
Mr Biden and Mr Sanders also split the vote in Texas, as the candidates' demographic advantages essentially cancelled each other out.
Edison, which compiles voter polls and live election results for media organisations including ABC News, CBS News, CNN, NBC News and Reuters, also found that as many as half of Super Tuesday voters made up their minds in the last few days.
The lion's share of those late deciders backed Mr Biden.
"It's a good night. It's a good night. And it seems to be getting even better." Joe Biden in major rebound on #SuperTuesday | https://t.co/yzRMDUjSt2 pic.twitter.com/2I7qTZHxcT
— RTÉ News (@rtenews) March 4, 2020
The results reflected how Mr Biden, who recovered from poor showings in the first two nominating contests, was benefiting from his dominant win in Saturday's South Carolina primary and endorsements he picked up from Senator Amy Klobuchar and Pete Buttigieg, a former South Bend, Indiana, mayor, two fellow moderates who dropped out of the race in recent days.
The polling found that late deciders ranged from five out of 10 voters in Virginia, Oklahoma, Minnesota and Massachusetts, to two out of 10 in Texas and California.
In almost every case, those late deciders broke at least 2-to-1 in favour of Mr Biden over Mr Sanders, who leads in national opinion polls in the Democratic race to find a challenger to Republican President Donald Trump in November's election.
In the case of Ms Klobuchar's home state of Minnesota, a little more than half of voters said they made up their minds in the "last few days," and about 49% of them went for Mr Biden, while 21% supported Mr Sanders.
