Joe Biden secured commanding victories in Michigan and two other states on Tuesday, cementing his position as a frontrunner in the Democratic presidential race.
This puts the former vice president as a challenger for Republican President Donald Trump in the November 3 election.
The next big election milestone begins in a week when 577 delegates are up for grabs.
Speaking in Philadelphia, Biden thanked Sanders and his supporters for their energy and passion. "We share a common goal and together we are going to defeat Donald Trump," he added.
"Winning means uniting America. Not sowing more division and anger."
A candidate needs the support of 1,991 delegates to secure the nomination.
US Epstein files release highlights Clinton, makes scant reference to Trump
US hits ISIS in Syria with large retaliatory strikes, officials say
Pakistan court hands Imran Khan, wife 17-year jail terms in another graft case
Seven elephants killed in India train accident
Israeli attack on school shelter in Gaza City kills 5 Palestinians
Nine injured in attack in Taipei, media reports
Putin offers no compromise on Ukraine, says EU 'robbery' failed
Bangladesh rocked by unrest after death of student leader
