The Democratic contenders pledge a path forward out of crises.

WILMINGTON, Del. — Joseph R. Biden Jr. and Kamala Harris made their debut as running mates in a high school gymnasium on Wednesday, pledging to lead the country out of the coronavirus crisis.

The first full day for the newly announced Democratic presidential ticket offered a glimpse of how two once-bitter rivals from opposite coasts and different generations will try to unite Americans around their candidacies. Projecting warmth toward each other, they sketched out a vision of recovery from the public health and economic catastrophes the nation is confronting — crises that, they argued, Mr. Trump has made worse at every turn with an extraordinarily divisive presidency.

Ms. Harris, a Californian who once served as attorney general of the state, made clear that part of her campaign role would be demonstrating her skills as a prosecutor to build a case against Mr. Trump and Vice President Mike Pence, methodically detailing what she cast as their failures in combating the coronavirus, reopening the economy and creating conditions under which schools could reopen safely this fall.

“Let me tell you, as somebody who has presented my fair share of arguments in court, the case against Donald Trump and Mike Pence is open and shut,” Ms. Harris said.

“This virus has impacted almost every country, but there’s a reason it has hit America worse than any other advanced nation,” she said. “It’s because of Trump’s failure to take it seriously from the start. His refusal to get testing up and running. His flip-flopping on social distancing and wearing masks. His delusional belief that he knows better than the experts. All of that is reason, and the reason, that an American dies of Covid-19 every 80 seconds.”