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.
Japan's 'Iron Lady' Takaichi forges historic election win
Hong Kong tycoon Jimmy Lai sentenced to 20 years in national security trial
Ukraine urges acceleration of peace talks
Japan's Takaichi set to secure big majority in lower house election
Thailand votes in three-way race as risk of instability looms
