Black voters in southern US states have been identified as the force that has propelled former vice-president Joe Biden back to frontrunner status in the Democratic party contest that will determine Donald Trump’s opponent in November’s presidential election.

Mr Biden’s presidential campaign spent the past month on the verge of collapse after disappointing finishes in the overwhelmingly white states that launched the Democratic primary.

As he watched the turmoil unfold from Gadsden, Alabama, Robert Avery thought the race would change dramatically when it moved into the South.

“He knows us, he cares about us,” the 71-year-old community organiser said.

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(PA Graphics)

“People have given us no credit as to knowing what’s going on or being involved, and that’s the furthest thing from the truth.”

It turns out Mr Avery’s instinct was right.

After a brutal February for Mr Biden, black voters throughout the South transformed Mr Biden’s White House bid over the course of three days.

A back-of-the-pack operation surged to frontrunner status powered by support from black voters, starting with Mr Biden’s commanding win on Saturday in South Carolina and coming into full focus on Super Tuesday as he racked up wins in Virginia, North Carolina, Alabama and Tennessee.

Mr Biden is now in a tight race with progressive rival Bernie Sanders, who emerged as the initial Democratic leader after strong performances in Iowa, New Hampshire and Nevada last month.

Activists said Mr Biden’s success is a reminder that the path to victory in the Democratic contest runs straight through their communities.

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Bernie Sanders is the main obstacle to Joe Biden winning the party nod (Charles Krupa/AP)

“You can’t win the Democratic presidential nomination without winning the South, and you can’t win the South without the black vote, and you can’t win the black vote without winning the black women’s vote,” said Melanie Campbell, president and chief executive of the National Coalition on Black Civic Participation.

“We believe all roads to the White House must come through the South.”

Black voters strongly aligned behind Mr Biden over other candidates, according to AP VoteCast surveys across eight Super Tuesday states.

In Alabama, where African American voters made up a majority of the Democratic primary electorate, roughly seven in 10 supported the former vice president who served under Barack Obama.

That support held across ages and gender.

In other states, like Texas, Virginia and North Carolina, black voters made up a smaller but sizeable share of Democratic primary voters.

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Joe Biden speaks in Los Angeles (Marcio Jose Sanchez/AP)

While a majority of both black men and women supported Mr Biden, Mr Sanders did pull about even with Mr Biden among black voters under 45.

Mr Biden will be looking to carry that momentum into next week’s six primaries that will include states like Michigan and Mississippi, which are also home to significant populations of black voters.

More than 78% of residents in Detroit, Michigan’s largest city, are African American.

The former vice president’s success also poses warning signs for Mr Sanders.

The Vermont senator’s 2016 presidential campaign was unsuccessful in part because he couldn’t line up enough support from black voters in the South to win the Democratic nomination.

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President Donald Trump will duel with the Democratic Party’s nominee in November (Evan Vucci/AP)

Since 2016, Mr Sanders has focused on building relationships with black leaders and voters. He frequently speaks of a multiracial coalition that will help him win the nomination and the White House.

But Tuesday’s results, combined with his distant second-place finish in South Carolina, suggest he could face similar challenges in 2020.

Mr Sanders has warned in recent days that the party’s elite establishment is aligning to thwart his campaign.

But Biden allies pointed to their success with a diverse set of voters on Tuesday to rebut that.

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Joe Biden served as vice-president to Barack Obama (Nick Potts/PA)

“I just did not know that African Americans in the South were considered part of the establishment,” said Louisiana Democratic Representative Cedric Richmond, Mr Biden’s campaign co-chairman.

“African Americans voters have made a conscious decision that we fought and earned through civil rights,” Mr Richmond said, because they understand the importance of “nominating a person that they know, nominating a person that can win”.