PORTSMOUTH, N.H. — Bernie Sanders is offering his long-awaited endorsement to Democratic rival Hillary Clinton, hoping to transfer the energy of his insurgent campaign into the party’s fight against Republican Donald Trump.

Sanders and Clinton were returning to New Hampshire today for the first time since he won a 22-point landside over Clinton in the state’s presidential primary. Democrats familiar with the plans said Sanders will publicly endorse Clinton’s White House bid after weeks of negotiations between the Clinton and Sanders campaigns. The Democrats spoke on condition of anonymity ahead of Sanders’ formal announcement.

The joint event at a Portsmouth high school will seek to project Democratic unity before Republicans formally nominate Trump next week in Cleveland. Clinton has campaigned with President Barack Obama and Massachusetts Sen. Elizabeth Warren in recent weeks, warning Democrats of the threat of a Trump presidency.

While Clinton took only a few days to endorse Obama after the primaries in 2008, Sanders has held out for the past month, seeking to influence the party’s platform and future. The Democratic candidates met at a Washington hotel in June and their campaigns have been in frequent contact since then.

Clinton last week rolled out proposals on college affordability and access to health care, winning praise from Sanders, and the platform agreed to last weekend in Florida includes many of Sanders’ priorities, including a $15 an hour minimum wage.

Recent polls have shown that many Sanders voters plan to back Clinton but have reservations about her honesty. Sanders has said he will do all he can to prevent Trump from winning the White House and the senator’s vouching for Clinton could help her with the independents, liberals and millennials who flocked to his primary campaign.

The Vermont senator saw his long-shot 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.

His campaign was powered by an impressive online fundraising machine that raised more than $200 million.