The Vermont senator joined Clinton at a campaign event in Portsmouth, New Hampshire.
He pledged that his "political revolution" would continue. But he acknowledged that she, and not he, had won the delegates to snag the nomination. That has been true for weeks, and Democrats have not-so-quietly been angling for him to drop out and endorse the former secretary of state.
"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 one percent," Sanders said, listing his reasons for endorsing her. "Hillary Clinton understands that if someone in America works 40 hours a week, that person should not be living in poverty."
The endorsement is the culmination of weeks of negotiations between the Clinton and Sanders campaigns since the final primaries in June.
"Having Sec. Clinton and Sen. Sanders campaign today for the first time proves that we are stronger together and that we are unified as Democrats in fighting to build an economy that works for everyone, not just those at the top," said NC Democratic Party Chair Patsy Keever. "Secretary Clinton and Senator Sanders have built the most progressive Democratic Party platform in history, and it will serve the people of North Carolina well because it's all about putting families first and bringing Americans together."
Sanders has pressed for Clinton's support of policy stances on higher education, health care and a $15-an-hour minimum wage.
Sanders beat Clinton in New Hampshire by a dominant 22-point margin last February. Many of his supporters have shifted to Clinton but the senator hopes to rally them as a way of preventing Trump from reaching the White House.
Donald Trump went after Sanders for his endorsement. Sanders, said Trump in a tweet, has "totally sold out to Crooked Hillary Clinton."
"Fans angry!" he added.
For weeks, Trump has been trying to woo Sanders backers frustrated with Washington politics by stressing his outsider candidacy.
The NC GOP also ripped Sanders for his endorsement of Clinton, calling it "laughable."
"Bernie Sanders' endorsement of Hillary Clinton is laughable after he repeatedly eviscerated her judgement, character and qualifications on the campaign trail," said NCGOP Chairman Robin Hayes. "There is no forgetting their contentious primary and Sanders' blistering criticisms of Clinton's ties to Wall Street and her reliance on big-money special interests to fund her campaign. While Sanders sells out to the same rigged system he fought so hard against, his supporters won't be so quick to back Clinton."
The Associated Press contributed to this report.
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