Bernie Sanders received the backing of veteran civil rights campaigner Jesse Jackson on Sunday, days ahead of a Michigan primary shaping up to be pivotal for his campaign for the Democratic presidential nomination. 

Sanders' White House bid was put on the back foot by Joe Biden's resurgence on Super Tuesday, with the former vice president outperforming expectations and winding up in a delegate lead after a slow start to his own campaign.

Biden, who picked up the endorsement of former rival Kamala Harris on Sunday, currently has 664 delegates to Sanders' 573, according to the AP, with at least 1,991 needed for a majority.

Michigan, one of six states to vote on Tuesday, is the most significant electorally because of its 125 delegates and its status as a swing state in the general election. The other five states – Idaho, Missouri, Mississippi, North Dakota and Washington – will allocate 227 delegates in total.

The vote in Michigan comes at a critical point for Sanders, who has campaigned in the Midwestern state for three days, with the race heading towards a series of contests which look favorable to Biden. In Florida, which has 219 delegates and votes on March 17, the former vice president holds a 37.5-point lead according to the RealClearPolitics polling average. 

Sanders beat Hillary Clinton in Michigan in 2016 despite trailing in the polls going into the primary, and will have to repeat that feat to best Biden, who leads by six points according to the latest Detroit News survey. That poll also featured four contenders who have dropped out in the race in the past week, most of whose supporters are likely to swing towards Biden.

"We're working as hard as we can because Michigan is very, very significant in terms of the primary process," Sanders said in a CNN interview on Sunday. "We hope to repeat the victory we had in 2016."

The former vice president has overwhelmingly won African American votes so far, a major problem for Sanders. The Vermont senator has been working to win over non-white voters in Michigan and hopes the backing of Jackson, a two-time contender for the Democratic nomination, can help.

"What Rev. Jackson understands is that we have to move aggressively to wipe out all forms of racism in this country and we need an economic agenda that speaks to the needs of working people, not just the billionaire class," he told CNN. "I think with Rev. Jackson  – I think we got a real boost in our campaign."

Jackson, who campaigned alongside Sanders in Grand Rapids, said Biden had not asked him for an endorsement.

Harris on Sunday became the latest former contender for the nomination to back Biden, saying "there is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honor, and decency to the Oval Office."

The endorsement from the California senator, who revealed in a video statement that she will campaign with Biden in Detroit on Monday, has added to speculation she is in the running to join the ticket as vice presidential nominee. 

Amy Klobuchar, Pete Buttigieg, Beto O'Rourke, Mike Bloomberg, Tim Ryan, Deval Patrick and John Delaney have all dropped out of the race and endorsed Biden. Marianne Williamson and Bill de Blasio have backed Sanders, while the influential Elizabeth Warren, Andrew Yang and Cory Booker have yet to pick a side.