Former vice president Joe Biden received the backing of another former rival for the Democratic presidential nomination on Monday ahead of a crucial head-to-head primary matchup with Vermont Senator Bernie Sanders in six US states.

"It's time for us to beat Donald Trump and it became very clear to me that Joe Biden is the right person to do that," New Jersey Senator Cory Booker told "CBS This Morning." "It's about time we start unifying as a party."

Booker, the charismatic former mayor of Newark who dropped out of the race in January, is the latest prominent African-American politician to back Biden's bid for the White House, his third following unsuccessful runs in 1988 and 2008.

California Senator Kamala Harris, who ended her campaign for the nomination in December, endorsed the 77-year-old centrist Biden on Sunday over the 78-year-old leftist Sanders, as has another also-ran, former Massachusetts governor Deval Patrick.

Booker and Harris, both touted as possible vice presidential picks for Biden, were to join him for a "Get Out the Vote" rally in Detroit, the largest city in the state of Michigan, on Monday evening. Biden has surged since scoring a decisive victory in the South Carolina primary on February 29, with endorsements from key African-American leaders helping him claim the frontrunner mantle in the race.

Democrats in six states go to the polls on Tuesday in what will be the first one-on-one duel between Biden and Sanders since all the other major candidates dropped out of the grueling nomination battle that began more than a year ago.

The biggest prize -- Michigan -- is shaping up as a must win for Sanders, who narrowly won the state over Hillary Clinton during his failed 2016 bid for the Democratic nomination. The latest polls for Michigan, however, have Biden with a lead of more than 20 points over Sanders.

Michigan's suffering auto industry received a major boost in 2008 from a massive intervention under the administration of former president Barack Obama but Sanders enjoys strong union support in the state. Democrats will also be voting in Idaho, Mississippi, Missouri, North Dakota and Washington to decide who will take on the Republican incumbent Trump in November.

Biden, who served for eight years as vice president to Obama, America's first black president, did well during last week's primaries in southern states with large African-American populations similar to Mississippi. The latest Missouri poll has Biden up there by nearly 19 points while he leads Sanders by a narrow two-point margin in Washington.

'One of the true heroes'

Sanders, desperate to reboot his campaign after losing 10 of the 14 states which voted on Super Tuesday last week, cancelled plans to speak in Mississippi, Missouri and Illinois in order to focus on Michigan.

The self-described democratic socialist gained the endorsement of civil rights leader Jesse Jackson on Sunday. But former Democratic candidates Booker, Harris, Patrick, Pete Buttigieg, Amy Klobuchar, Michael Bloomberg and Beto O'Rourke have swung behind Biden, seeing a moderate as having the best chance of defeating Trump.

Massachusetts Senator Elizabeth Warren, another also-ran with a sizeable following on the left, has yet to decide whether to endorse Biden or Sanders. Jackson endorsed Sanders at a rally on Sunday in Grand Rapids, Michigan, repaying the Vermont senator for having supported his 1988 bid for the White House. "I stand with Bernie Sanders today because he stood with me," Jackson said. "I stand with him because he stands with you."

"Sanders has a better chance at beating Trump than Biden does," Sara Long, 25, told AFP as she stood in line for the rally. "I think that a lot of his views are more progressive, and they're what this generation is looking for."

Sanders touted Jackson's endorsement, telling a rally in Ann Arbor, Michigan, that he was "one of the true heroes of modern America."

Sanders called on his supporters to come out in force on Tuesday.  "If you get your friends in the political process, there's no stopping us," he said. Biden's Super Tuesday surge brought an influx of donations -- $22 million in the past few days, his campaign said in a statement Sunday. It said $12 million would be spent on hiring new staff and launching a major media campaign in battleground states.