Mr Bernie Sanders has officially endorsed former US secretary of state Hillary Clinton in a bid to unite the Democratic Party and clear the path for her nomination at the Democratic national convention in less than two weeks.
The Vermont senator, who had challenged the notion that Mrs Clinton was the inevitable party nominee, appeared with Mrs Clinton on Tuesday at a rally in Portsmouth, New Hampshire - the state that gave Mr Sanders his first win during the primaries and which is a critical swing state in November's presidential election.
Speaking to the crowd, Mr Sanders said: "Secretary Clinton has won the Democratic nominating process and I congratulate her for that. She will be Democratic nominee for president and I intend to do everything I can to make certain she will be the next president of the United States."
He went on to outline his reasons for endorsing his former rival, highlighting their shared goals of lowering student debt, increasing access to healthcare and creating jobs in America.
"This campaign is about the needs of the American people and addressing the very serious crises that we face, and there is no doubt in my mind that, as we head into November, Hillary Clinton is far and away the best candidate to do that," he said.
Mr Sanders' campaign has said he will continue to appear at events across the nation in support of Mrs Clinton. The endorsement is an important symbol of party unity and comes after a month of intense discussions between Mr Sanders and Mrs Clinton's camp.
"It will show that the two big delegate holders are united behind one nominee. It will help make the convention less about the future of the Democratic Party platform and more about electing Clinton and defeating (Donald) Trump," said Dartmouth College associate professor of government Joseph Bafumi.
After negotiations with the Clinton camp, the Sanders campaign successfully advanced a more progressive party platform - a formal set of party principles - and gained policy concessions in areas such as affordable healthcare and college debt.
"The preliminary platform of the Democratic Party is the most liberal platform they have ever produced... Sanders supporters are responsible for pushing Clinton further to the left than I believe she wanted to go," said Mr Andrew Smith, associate professor of practice in political science and director of the University of New Hampshire Survey Centre.
Last week, for example, Mrs Clinton unveiled her plan to offer free in-state college tuition for families making US$125,000 (S$168,000) or less.
However, Mr Sanders failed to win support for blocking a vote in Congress on the Trans-Pacific Partnership trade deal.
When Mrs Clinton addressed the crowd on Tuesday, she thanked Mr Sanders for his endorsement and acknowledged his success in energising young voters and bringing them into the political process.
She also echoed Mr Sanders' message of unity.
"With your help we are joining forces to defeat Donald Trump, win in November and, yes, together build a future we can all believe in," she said.
But in a swipe at Mr Sanders' endorsement, Mr Trump, the presumptive Republican nominee, tweeted: "Bernie Sanders endorsing Crooked Hillary Clinton is like Occupy Wall Street endorsing Goldman Sachs."
Mrs Clinton has extended her lead over Mr Trump to 13 percentage points in a Reuters/Ipsos opinion poll released on Tuesday, up from 10 points at the end of last week.
She is hoping the Portsmouth rally will win over more of Mr Sanders' supporters, some of whom shouted and carried Sanders signs, drowning out Mrs Clinton's backers.
