Bernie Sanders has endorsed Hillary Clinton as the Democratic nominee in the race for the White House.
The Vermont senator stood side-by-side with his rival as he backed Mrs Clinton at a rally in Portsmouth, New Hampshire.
He pledged that his "political revolution" would continue, but acknowledged that Mrs Clinton had won the delegates required to snag the nomination.
This has been true for weeks, and Democrats have been angling for Mr Sanders to drop out of the campaign and endorse the former secretary of state.
Mr Sanders promised to work to help Mrs Clinton win in November, calling her "far and away the best candidate" to confront the challenges facing America.
Presumptive Republican nominee Donald Trump has been trying to draw Sanders supporters towards his campaign.
Mr Sanders called for unity just two weeks shy of the Democratic National Convention.
The self-described democratic socialist, whose calls for a "political revolution" energised millions of American voters, congratulated Mrs Clinton for winning the nomination and vowed to do everything he can to help her beat Donald Trump.
Listing his reasons for the endorsement, Mr Sanders said: "Hillary Clinton understands that we must fix an economy in America that is rigged and that sends almost all new wealth and income to the top 1%.
"Hillary Clinton understands that if someone in America works 40 hours a week, that person should not be living in poverty."
Democrats have coalesced around Mrs Clinton's candidacy since she defeated Mr Sanders in primaries last month in California and five other states, led by endorsements from president Barack Obama, vice president Joe Biden and others.
Mr Sanders has spent the past month seeking to influence the party's platform at the Philadelphia convention and promote electoral reforms, including allowing independents to participate in future primaries.
The platform includes many of his proposals, including a 15 dollars per hour (€13.50) minimum wage, tougher restrictions on Wall Street and an end to the death penalty.
The event at a Portsmouth high school sought to project Democratic unity before Republicans formally nominate Mr Trump next week in Cleveland - but some Sanders supporters in the crowd did not appear to be ready to move on.
Chants of "Bernie" broke out in the gymnasium while opening speakers addressed the crowd, prompting Mrs Clinton's backers to chant: "Unity".
When senator Jeanne Shaheen, a Clinton supporter, told the audience: "We need to elect Hillary," some Bernie supporters stood and shouted "No," which was followed by chants of "Hillary" from other parts of the crowd.
The Vermont senator saw his longshot bid for the White House quickly catch fire in 2015 at large-scale rallies where he denounced income inequality, the influence of Wall Street and the role of big money in politics - all part of a system he described as "rigged".
Mr Sanders was powered by an impressive online fundraising machine which raised more than $200m (€181m) and threatened Mrs Clinton's once overwhelming lead in the Democratic primaries. He drew upon the support of college students, independents and white voters drawn to his anti-establishment message.
His endorsement could help bring some of those supporters into the fold for Mrs Clinton as she prepares to face off with Mr Trump in the autumn.
Mr Trump, who previously said Mr Sanders had been "treated terribly by the Democrats" and should have run as an independent, wasted no time in going after the Vermont senator.
"Bernie Sanders, who has lost most of his leverage, has totally sold out to Crooked Hillary Clinton," Mr Trump wrote on Twitter.
Bernie Sanders, who has lost most of his leverage, has totally sold out to Crooked Hillary Clinton. He will endorse her today - fans angry!— Donald J. Trump (@realDonaldTrump) July 12, 2016
"Sanders was not true to himself and his supporters."
I am somewhat surprised that Bernie Sanders was not true to himself and his supporters. They are not happy that he is selling out!— Donald J. Trump (@realDonaldTrump) July 12, 2016
Donald Trump's campaign said Mr Sanders "is now officially part of a rigged system".
Senior policy adviser Stephen Miller said the Vermont senator is "endorsing one of the most pro-war, pro-Wall Street, and pro-off-shoring candidates in the history of the Democratic Party".
He added: "The candidate who ran against special interests is endorsing the candidate who embodies special interests."