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.
Emergency GCC-EU meeting to discuss Iranian aggression
Omani navy rescues cargo ship crew after missile strike
UN 'deeply disturbed' by strike on Iran school that killed 160 children
Saudi confirms attempted attack on oil refinery, no damage
At least four killed in Israeli strike on building in Lebanon's Baalbek
