ANN ARBOR, United States — Democratic presidential hopefuls Joe Biden and Bernie Sanders secured crucial endorsements Sunday from prominent black supporters just days ahead of the first round of voting to put them in a head-to-head contest.

Senator Kamala Harris, a former Democratic candidate herself, endorsed Biden, while Sanders won the backing of civil rights leader Jesse Jackson as the rival candidates competed for African American support – a key demographic in the fight for the party's nomination.

Voters in six states go to the polls on Tuesday, a week after the "Super Tuesday" elections dramatically reversed the two men's fortunes, snatching the front-runner's title from Sanders and revitalizing Biden, who now holds a lead in delegates to the nominating convention.

Biden did well on Super Tuesday in Southern states with large black populations, states similar to Mississippi, which votes Tuesday. And in Missouri, a Midwestern state also voting 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.

"Joe has been there for Michigan when our back was against the wall," Governor Gretchen Whitmer said on Sunday, during an appearance at a majority-black church in Detroit.

Michigan's critically suffering auto industry received a major boost in 2008 from a massive intervention under the administration of Barack Obama and Biden.

But Whitmer predicted a close race on Tuesday; Sanders has a large organisation in Michigan with considerable union support.

Also voting Tuesday are Idaho, North Dakota and Washington state.

Sanders focuses on Michigan

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

Jackson endorsed Sanders at a rally in Grand Rapids, Michigan, repaying the self-described democratic socialist 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, said 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," she said.

Sanders touted Jackson's endorsement, telling supporters at the 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.

Harris said she was backing Biden, a centrist who touts his ability to work with Republicans, because she believes he can best unify the country going into the crucial November elections against Donald Trump.

"I am with great enthusiasm going to endorse Joe Biden for president of the United States," she said in a videotaped statement posted on Twitter.

Eight other former Democratic candidates -- including Pete Buttigieg, Amy Klobuchar and Beto O'Rourke – had earlier endorsed Biden.

He later tweeted his thanks to Harris, saying, "You've spent your whole career fighting for folks who've been written off and left behind."

The endorsement was something of a reversal; Harris had sharply chastised Biden in a televised debate last June over his warm words for past segregationist senators and his opposition in the 1970s to busing to integrate US schools.

But many African American leaders have since swung behind Biden, helping to resurrect his once flagging campaign.

Blacks' ‘best chance’

Sanders has had trouble attracting black voters -- Biden won more than four black votes in South Carolina for each one favouring Sanders -- making the endorsement from Jackson all the more significant.

Jackson questioned whether moderate policies would benefit African Americans.

"A people far behind cannot catch up choosing the most moderate path," said Jackson.

"The most progressive social and economic path gives us the best chance to catch up, and Senator Bernie Sanders represents the most progressive path. That's why I choose to endorse him today." — AFP