Booker, who dropped out of the nominating contest in January after centering his campaign on themes of love and justice, was one of several candidates who struggled to emerge as an alternative to Biden in what began as a historically diverse and crowded Democratic field.

As the primary fight intensified, Booker frequently challenged Biden on matters of race, criticizing the former vice president for statements he made about working with segregationist lawmakers in the Senate and arguing that Biden was ill-equipped to effectively discuss America's centuries-old racial wounds.

Booker also expressed concerns about Biden's gaffe-prone oratorical style and unsteady debate performances, remarking that Democrats "have to have a nominee that's up to this challenge" and asserting that voters were worried about his "ability to carry the ball all the way across the end line without fumbling."

But Booker's endorsement Monday reflects the current realities of the primary race, which has effectively narrowed to Biden and Sanders, a self-identified democratic socialist who many in the party fear is too left-wing to head up the ticket in November and could negatively impact down-ballot Democratic candidates in House and Senate races.

Sanders and his supporters have accused senior members of the Democratic establishment of lining up to thwart his campaign in the aftermath of Biden's impressive performance in last week's Super Tuesday primaries — a charge echoed by President Donald Trump as he seeks to sow division among Democrats ahead of the general election.