Joe Biden, accompanied by his wife Dr. Jill Biden, speaks to California voters during a Super Tuesday election night party at Baldwin Hills Recreation Center in Los Angeles, California on Tuesday March 3, 2020. (Photo by Melina Mara/The Washington Po WASHINGTON (AP) - Joe Biden formally clinched the Democratic presidential nomination Friday, 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.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.