Image copyright Getty Images

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.

Kamala 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."

Media playback is unsupported on your device Media caption What unites these two bitter rivals?

The endorsement comes just ahead of another series of primary contests. Six states will vote on Tuesday, including Michigan - a key battleground state that narrowly went to Mr Trump in 2016.

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.

Another former black candidate, ex-Massachusetts Governor Deval Patrick, has also backed Mr Biden, as did erstwhile rivals Mike Bloomberg, Tim Ryan and John Delaney.

Despite Mr Booker's fulsome endorsement for Mr Biden, he has not always been so supportive.

After a live TV debate in September, the New Jersey senator seemed to express doubts about the 77-year-old former vice-president's mental acuity.

Mr Booker said: "There were a lot of moments where a number of us were looking at each other on stage when [Biden] tends to go on sometimes."

He added: "There are definitely moments where you listen to Joe Biden and you just wonder."

Read more on Joe Biden

Image copyright Getty Images

While opinion polls have raised questions about Mr Sanders' ability to connect with black voters, the Vermont senator picked up a coveted endorsement on Sunday from civil rights leader Jesse Jackson.

Mr Sanders has been endorsed by former rivals, Marianne Williamson and Bill de Blasio.

The Democratic field once had more than two dozen candidates.

Now effectively a two-man race, the eventual winner will be anointed at the party convention in July in Milwaukee, Wisconsin.