Kamala Harris has started her first full day of campaigning in the White House race as she vowed to win November’s presidential election.
The US vice president and expected Democrat nominee made the remarks on X a day after Mr Biden announced he was quitting the 2024 contest and throwing his weight behind his number two.
Ms Harris said she was going to visit workers at the campaign headquarters in Delaware.
She wrote: “It’s the first full day of our campaign, so I’m heading up to Wilmington, DE later to say ‘hello’ to our staff in HQ.
“One day down. 105 to go. Together, we’re going to win this.”
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Meanwhile, at an event for college athletes in the White House grounds, Ms Harris said Mr Biden’s legacy of accomplishment was unmatched in modern history.
Speaking for the first time in public since entering the presidential race, she said Mr Biden had “already surpassed the legacy of most presidents who have served two terms in office”.
“I am [a] first-hand witness that every day our president Joe Biden fights for the American people, and we are deeply grateful for his service to this nation,” she said.
He is currently recovering from COVID with Ms Harris saying he is “feeling much better and recovering fast”.
Ms Harris recounted how she first knew him through his son Beau when they worked together as attorney generals.
She said: “The qualities that Beau revered in his father are the same qualities that I have seen every day in our president. His honesty, his integrity, his commitment to his faith and his family, his big heart and his love, deep love of our country.”
Ms Harris, 59, is the only declared Democratic candidate so far to be the party’s nominee ahead of the election against 78-year-old Republican Donald Trump.
She has been working to secure endorsements from a majority of its delegates and if she wins the nomination, she will need to pick a running mate.
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Michigan governor Gretchen Whitmer, seen as a possible rival for the Democratic nomination after President Biden‘s exit, has backed Ms Harris in a post on X, saying the vice president had her full support.
Several other potential Democratic challengers, including California governor Gavin Newsom and Kentucky governor Andy Beshear, have also supported Ms Harris’ bid.
But former House speaker Nancy Pelosi and ex-president Barack Obama have not announced endorsements, although both praised the current president.
Mr Biden’s campaign has formally changed its name to “Harris for President”, showing that she is inheriting his political operation of more than 1,000 staff and a war chest that was almost $96m (£74m) at the end of June.
Campaign spokeswoman Lauren Hitt said on Monday that Ms Harris had raised $49.6m (£38.3m) in donations in the 15 hours after Mr Biden’s endorsement.
The 81-year-old politician said he was no longer standing for re-election after weeks of pressure from within his own party amid concerns about his health and fitness to serve another four years in office.
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The Trump campaign has been preparing for Ms Harris’s possible rise for weeks, sources told the Reuters news agency, and planned to try to tie her closely to Mr Biden’s policies on immigration and the economy.
Mr Biden, who is the oldest person ever to have occupied the Oval Office, has said he will remain as president until his term ends on 20 January 2025.