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.
Trump touts $5,000 payout if Republicans win as Vance waits in wings
Search expands after Philippine ferry fire kills five and leaves dozens missing
Trump says Iran war to end after US midterm elections
At least 20 killed in cargo vessel fire at China shipyard, state media reports
US military says it struck vessel in Caribbean, killing 3
At least 14 killed in explosion at weapons depot in Syria
US hits five Iranian oil tankers after attack on Navy ships
UK gives air traffic provider a week to probe outage that halted flights
