(AP) - Bernie Sanders is offering his long-awaited endorsement of Democratic rival Hillary Clinton, hoping to transfer the energy of his supporters into the party's fight against Republican Donald Trump.
The Vermont senator is joining Clinton at their first joint campaign event in Portsmouth, New Hampshire.
He told the crowd, `She must become our next president' .
The endorsement is the culmination of weeks of negotiations between the Clinton and Sanders campaigns since the final primaries in June.
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.
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
Meanwhile, Trump is going after Sanders for his endorsement of Clinton.
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.
Sanders has vowed to do everything he can to defeat Trump in November. Recent polling suggests that 85 percent of his supporters have said they plan to back Clinton.
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Also, Clinton's campaign will be topping off her presidential nomination with an event on Independence Mall, the day after the Democratic National Convention ends.
Clinton's campaign has been issued a permit from the National Park Service for a public assembly on July 29, according to a park service schedule of events during the week of the convention. The permit includes a stretch of park near the Liberty Bell and Independence Hall.
The location can host sizable crowds: Pope Francis delivered a speech on religious freedom and immigration in front of about 40,000 people at Independence Hall during his Philadelphia visit in September.
The convention is in less than two weeks -- July 25-28 -- at the Wells Fargo Center.