WASHINGTON (AP) — Joe Biden has formally clinched the Democratic presidential nomination, setting him up for a bruising challenge to President Donald Trump that will play out against the unprecedented backdrop of a pandemic, economic collapse and civil unrest.

“It was an honour to compete alongside one of the most talented groups of candidates the Democratic party has ever fielded,” Biden said in a statement Friday night, “and I am proud to say that we are going into this general election a united party.”

The former vice-president has effectively been his party’s leader since his last challenger in the Democratic primary, Bernie Sanders, ended his campaign in April. But Biden pulled together the 1,991 delegates needed to become the nominee Friday after seven states and the District of Columbia held presidential primaries Tuesday.

Biden reached the threshold three days after the primaries because several states, overwhelmed by huge increases in mail ballots, took days to tabulate results. A team of analysts at The Associated Press then parsed the votes into individual congressional districts. Democrats award most delegates to the party’s national convention based on results in individual congressional districts.

Biden now has 2,000 delegates, with contests still to come in eight states and two US territories.

The moment was met with little of the traditional fanfare as the nation confronts overlapping crises. While Biden has started to venture out more this week, the coronavirus pandemic has largely confined him to his Wilmington, Delaware, home for much of the past three months.

The country faces the worst rate of unemployment since the Great Depression. And civil unrest that harkens back to the 1960s has erupted in dozens of cities following the death of George Floyd, a black man who died when a white Minneapolis police officer pressed his knee into Floyd’s neck for several minutes even after he stopped moving and pleading for air.

It’s a confluence of events that no US leader has faced in modern times, made all the more complicated by a president who has at times antagonised the protesters and is eager to take the fight to Biden.

“This is a difficult time in America’s history,” Biden said Friday night. “And Donald Trump’s angry, divisive politics is no answer. The country is crying out for leadership. Leadership that can unite us. Leadership that can bring us together.”

Biden spent 36 years in the Senate before becoming Barack Obama’s vice president. This is 77-year-old Biden’s third bid for the presidency and his success in capturing the Democratic nomination was driven by strong support from black voters.

He finished an embarrassing fourth place in the overwhelmingly white Iowa caucuses that kicked off the nomination process in February. Biden fared little better in the New Hampshire primary, where his standing was so low that he left the state before polls closed on election night to instead rally black voters in South Carolina.

His rebound began in the more diverse caucuses in Nevada but solidified in South Carolina, where Biden stomped Sanders, his nearest rival, by nearly 29 points. He followed that with a dominant showing three days later during the Super Tuesday contests, taking 10 of the 14 states.

Biden’s strong showing in states such as North Carolina, Virginia, Tennessee and Texas reinforced his status as the preferred Democratic candidate of African American voters — but the relationship has not been without its strained moments. After a tense exchange with an influential black radio host, Biden took sharp criticism for suggesting that African American voters still deciding between him and Trump “ain’t black”.

That comment, and protests that have spread nationwide, have increased pressure on Biden to pick an African American running mate. He has already committed to picking a woman as a vice-presidential candidate.

Black voters are unlikely to back Trump over Biden by a wide margin. A recent Fox News poll shows just 14 per cent of African Americans who are registered to vote have a favourable opinion of the president compared with 75 per cent who favourably view Biden.

But Biden must ensure that black voters are motivated to show up to the polls in November, especially in critical swing states that narrowly went for Trump in 2016.

At one point, the Democratic primary included dozens of candidates of different races, genders and generations and an openly gay man. The contest was dominated by debate over unapologetically progressive ideas, including fully government-funded health care under “Medicare for All” and a sweeping proposal to combat climate change known as the “Green New Deal”.

Biden prevailed by mostly offering more moderate approaches that he argued would make him more electable against Trump.

He refused to budge on his rejection of universal health care and some of the Green New Deal’s most ambitious provisions to combat climate change.

Since clinching the nomination, however, Biden has worked to build his appeal among progressives, forming joint task forces with Sanders’ campaign to find common ground on key issues like health care, the economy and the environment. Biden has also embraced a plan to forgive millions of Americans’ student debt, meaning that he clinches the nomination as easily the most liberal standard-bearer the Democratic Party has ever had.

Biden’s embrace of his party’s left flank could help him consolidate a Democratic base that remained deeply divided after the 2016 primary and ultimately hurt Hillary Clinton in her defeat to Trump. But it could also undermine Biden’s attempts to rebuild the Obama coalition, which is often loosely defined as minorities and young people, as well as educated Americans and some working-class voters.

The former vice-president has sought, since announcing his candidacy, to cast the election as a battle “for the soul of the nation”, and promised to restore order and dignity to the White House while rehabilitating the US image on the world stage. Such an approach, though, necessarily focuses on being more of an alternative to Trump than offering radically new political ideas. And that further underscores Biden’s difficult task of trying to unite his party’s base while appealing to voters from far beyond it.

“I am going to spend every day between now and November 3rd fighting to earn the votes of Americans all across this great country,” Biden promised Friday, “so that, together, we can win the battle for the soul of this nation, and make sure that as we rebuild our economy, everyone comes along.”