Former VP Joe Biden now has a substantial delegate lead over Bernie Sanders after winning in Michigan, Missouri, Mississippi and Idaho.
This comes after her poor performance on Super Tuesday.
Joe Biden won a crucial victory in South Carolina's Democratic primary, but will it be enough to keep his campaign alive heading into Super Tuesday?
Sanders won an overwhelming victory in Nevada, coming far ahead of his rivals and cementing his place as the opposition party's frontrunner.
If Sanders is the nominee, some Democrats worry Trump will hammer him on his long-buried words in defence of governments in Nicaragua, Cuba and the USSR.
Only 62% of the results had been released after a problem with an online voting app.
25 candidates are standing for the Democrats’ nomination and a further 10 will be heard in a second debate on Thursday night.
Bernie Sanders has publically endorsed his older brother Larry in his campaign to represent former UK Prime Minister David Cameron's constituency.
The deep divisions within the US Democratic party were on clear display on the first day of the party convention in Philadelphia on Monday.
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The Democratic Convention also brought out the support of several entertainment and sports stars for Hillary Clinton’s presidential bid.