Bernie Sanders has promised to work with the presumptive Democratic presidential nominee, Hillary Clinton, but has stopped short of endorsing her.
The Vermont senator told his supporters in a speech that it was vital that they stop the Republican candidate, Donald Trump, from winning the White House.
He also promised to continue working to “transform” the Democratic Party.
Mr Sanders has resisted ending his campaign since Mrs Clinton effectively sealed the nomination last week.
He won 22 states primaries and caucuses, receiving more than 12 million votes.
But the former secretary of state won 16 million votes and enough pledged and super delegates to pass the threshold needed for nomination at the Democratic convention in July.
President Barack Obama and Vice-President Joe Biden have both endorsed Mrs Clinton.