WASHINGTON — Joe Biden picked up another key endorsement on Monday in his race for the White House, this time from Senator Cory Booker, a prominent African American leader and former rival for the Democratic nomination.

Booker — the latest in a string of former candidates to endorse Biden — announced his decision as the former vice president goes head to head against Bernie Sanders on Tuesday in the next round of Democratic primary contests.

"Joe Biden won't only win — he'll show there's more that unites us than divides us," Booker said on Twitter. "He'll restore honor to the Oval Office and tackle our most pressing challenges."

Biden has surged since scoring a decisive victory in South Carolina February 29, with endorsements from key African American leaders helping to propel him to the lead of the Democratic race ahead of Sanders.

Biden did well on Super Tuesday in Southern states with large black populations, states similar to Mississippi, which votes on Tuesday. And in Missouri, a Midwestern state also voting on Tuesday, one recent poll gives him a 22-point lead.

That makes Michigan, the day's biggest prize, an almost must-win for Sanders. A survey in that north-central state last week gave Biden a six-point advantage.

Sanders, a self-described democratic socialist, gained the endorsement of civil rights leader Jesse Jackson on Sunday.

But many high profile former candidates — Pete Buttigieg, Amy Klobuchar, Michael Bloomberg, Beto O'Rourke, and, on Sunday, Kamala Harris — have swung behind Biden, seeing a moderate as having the best chance of defeating Donald Trump in November.

Elizabeth Warren, another also-ran with a sizeable following on the left, has yet to decide whether to endorse either Biden or Sanders.