She praises Biden, saying she is “deeply grateful for his service to our nation”.
More leading Democrats are backing Harris as the party’s new presidential nominee, as her campaign sees a surge in donations.
Some key Democrats haven’t yet endorsed Harris after Biden dropped out.
Biden had faced weeks of increasing calls to step aside – he’ll stay on as president for the rest of the term.
But Donald Trump has echoed other Republicans, saying, “if he can’t run for office, he can’t run our country”
Trump’s running mate JD Vance will make his first solo appearance on the campaign trail later, with a speech in Ohio.
