A resurgent Joe Biden has swept to victory across the country on so-called Super Tuesday.
The former vice president recorded wins in the upper Midwest and African American strongholds in the South, in a dramatic offensive against progressive rival Bernie Sanders.
Mr Sanders, considered the front-runner in the Democratic presidential contest, was hoping to tap into delegate-rich Western states to maintain his lead.
The two Democrats, lifelong politicians with starkly different visions for America’s future, were battling for delegates as 14 states and one US territory held a series of high-stakes elections that marked the most significant day of voting in the party’s 2020 nomination fight.
The clash between Biden and Sanders, each leading coalitions of disparate demographics and political beliefs, peaked on a day that could determine whether the Democrats’ 2020 nomination fight will stretch all the way to the party’s July convention or be decided much sooner.
“One of us in this race led the opposition to the war in Iraq,” Mr Sanders said in a speech drawing a contrast with Mr Biden.
“You are looking at him. Another candidate voted for the war in Iraq.”
Mike Bloomberg’s sole victory was in the territory of American Samoa.
The billionaire former New York mayor will reassess his campaign on Wednesday, according to a person close to his operation who spoke on the condition of anonymity.