Bernie Sanders has officially endorsed Hillary Clinton as the Democratic nominee for the US Presidency.
For the first time, the Vermont senator stood side-by-side with Clinton at a rally in New Hampshire.
He acknowledged his race for the White House was over and is vowing to do everything he can to help Clinton beat Donald Trump.
"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 told a raucous crowd in Portsmouth, New Hampshire.
With Clinton nodding in agreement, Sanders put their bitter primary campaign behind them and said she would take up the fight to ease economic inequality, make college more affordable and expand healthcare coverage for all Americans.
"I intend to do everything I can to make certain she will be the next president of the United States," the Vermont senator said.
His endorsement, coming five weeks after Clinton became the presumptive Democratic presidential nominee, brought the most prominent holdout in the party's liberal wing into her camp.
Sanders threw Clinton his support less than two weeks before the Democratic National Convention in Philadelphia, where she is to be formally nominated.
"I can't help but reflect on how much more enjoyable this election will be now that we are on the same side," Clinton said.
"Thank you, Bernie, for your endorsement, but more than that, thank you for your lifetime of fighting injustice."
In a statement, the Trump campaign said Sanders was now officially part of the rigged system he had criticised during his long primary battle with Clinton.
"Bernie's endorsement becomes Exhibit A in our rigged system - the Democrat Party is disenfranchising its voters to benefit the select and privileged few," said Stephen Miller, a senior policy adviser to Trump.
Clinton hopes the joint appearance will help her win over Sanders supporters, some of whom carried Sanders signs into the rally and frequently drowned out her backers.
In recent Reuters/Ipsos polling, only about 40 percent of Sanders backers said they would back Clinton, and the crowd at Tuesday's rally made it clear she still had work to do.
"I am absolutely certain I will not vote for Hillary Clinton," said Gale Bailey, a Sanders supporter and an unemployed graphic designer from Rochester, New Hampshire, who attended the rally in a Sanders T-shirt.
"She's a crook, and I'm not going to vote for a crook," Bailey said, adding that she would write in Sanders' name on the November ballot.