WASHINGTON -- By formally clinching the Democratic presidential nomination on Friday, Joe Biden is set for a bruising challenge to President Donald Trump that will play out against the unprecedented backdrop of a pandemic, economic collapse, and civil unrest. 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 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. Teams of analysts 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 1,993 delegates, with contests still to come in eight states and three 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 antagonized the protesters and is eager to take the fight to Biden. 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 9 of the 13 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.” -- Agencies