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Democratic presidential hopeful Joe Biden has picked up another prize endorsement from a former rival, Cory Booker.
The New Jersey senator tweeted: “The answer to hatred & division is to reignite our spirit of common purpose.”
He added that Mr Biden “won’t only win – he’ll show there’s more that unites us than divides us”.
Mr Biden and Bernie Sanders are vying to be the Democratic nominee who will face Donald Trump in November.
Ms Harris, who was the only black woman in the Democratic field before she ended her campaign in December, endorsed Mr Biden on Sunday.
The California senator’s thumbs-up has ignited speculation about whether Mr Biden might select her as a running mate if he wins the Democratic presidential nomination.
Both Ms Harris and Mr Booker, who is also African American, are expected to appear alongside Mr Biden at a rally in Detroit, Michigan, on Monday night.
Mr Booker will also reportedly campaign with Mr Biden in Flint, Michigan, earlier in the day.
In an email, Mr Booker – who dropped out of the race in January – said: “Joe is building the kind of campaign that will do more than remove one guy from one office.
“He will lead the Democratic Party to victory in races up and down the ballot across the country this November.”
A week ago, three other former Biden rivals – Senator Amy Klobuchar of Minnesota, Pete Buttigieg, a former mayor from Indiana, and former Texas congressman Beto O’Rourke – all endorsed the now Democratic frontrunner.