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 Mrs Clinton won 16 million votes and enough pledged and super delegates to pass the threshold needed for nomination at a party convention next month.
In an speech broadcast online from his hometown of Burlington on Thursday, Mr Sanders signalled to his supporters he was winding down his campaign.
“The major political task that we face in the next five months is to make certain that Donald Trump is defeated and defeated badly, and I personally intend to begin my role in that process in a very short period of time,” he said.
But, the senator added, defeating Mr Trump could not be their only goal.
“We must continue our grassroots effort to create the America that we know we can become.
“And we must take that energy into the Democratic National Convention on 25 July in Philadelphia, where we will have more than 1,900 delegates.”
Mr Sanders said it was “no secret” that he and Mrs Clinton, who met on Tuesday night, had “strong disagreements on some very, very important issues”.
“I look forward, in the coming weeks, to continued discussions… to make certain that your voices are heard and that the Democratic Party passes the most progressive platform in its history and that Democrats actually fight for that agenda,” he added.
“I also look forward to working with Secretary Clinton to transform the Democratic Party so that it becomes a party of working people and young people.”
He stressed that his vision for the future was not a fringe or radical idea, but something that “millions of Americans believe in and want to see happen”.
In a separate development on Thursday, Mrs Clinton won the endorsement of the AFL-CIO, the largest union federation in the US with 12.5 million members.