Bernie Sanders has endorsed Hillary Clinton to be the United States Democratic Party's candidate for the presidency.
The Vermont senator and former Democratic presidential candidate appeared with Mrs Clinton at a campaign event in New Hampshire.
Mrs Clinton and Mr Sanders had been negotiating since she all but guaranteed the nomination in June.
The democratic socialist said Mrs Clinton understood the need to fix economy that sent almost all of its new wealth and income to the richest one percent of Americans.
Mr Sanders hopes to have a large influence on the Democratic platform.
"She will be the 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 said.
"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."
Some Sanders supporters booed at the joint rally and Mr Sanders appeared to motion them to stop. At the end of Mrs Clinton's remarks, the two shared a hug and smiled.
Mr Sanders defeated Mrs Clinton in New Hampshire's primary contest in February.
"Thank you for your lifetime of fighting injustice," said Mrs Clinton, and credited Mr Sanders for bringing new Americans into the political process.
"I'm proud to be fighting behind you... it's a time for all of us to stand together."
She spoke about student debt, Wall Street, raising the minimum wage, the US tax code, reforming policing, strengthening the middle class, climate change, health care and reducing gun violence.
His supporters have largely decided to support Mrs Clinton in an effort to stop presumptive Republican nominee Donald Trump from winning the White House.
Mr Sanders stayed in the Democratic race far longer than many expected, for weeks after it became clear he would not secure the delegates needed to win the nomination.
He has pressed Mrs Clinton to support his views on higher education, health care and the minimum wage.
"It is no secret that Hillary Clinton and I disagree on a number of issues. That's what this campaign has been about. That's what democracy is about," said Mr Sanders.
"But I am happy to tell you that... there was a significant coming together between the two campaigns and we produced, by far, the most progressive platform in the history of the Democratic Party."
Mr Trump, who has been trying to court Sanders supporters, wrote in a tweet that Mr Sanders has "totally sold out to Crooked Hillary Clinton. Fans angry!"
"The candidate who ran against special interests is endorsing the candidate who embodies special interests," his campaign said in a release.