“Looking back over our history, you see that it’s been in the most difficult circumstances that the soul of our nation has been forged,” Mr. Biden said, speaking directly to the camera from a stage at the Queen, a historic theater in Wilmington, Del., where he stepped into a void left for him by President Trump, who has been rarely seen since the election.

In an implicit repudiation of Mr. Trump, who has dismissed the coronavirus as the flu and mocked people who wear masks, Mr. Biden urged Americans to see it as their patriotic duty to fight the pandemic together by taking the proper precautions. “I know the country has grown weary of the fight,” he said. “We need to remember we’re at war with the virus, not with one another, not with each other.”

As he urged Americans to wear face masks, practice social distancing and limit the size of group gatherings, especially around the holidays, he noted: “None of these steps we’re asking people to take are political statements. Every one of them is based on science, real science.”