Biden, Seizing on Masks as a Campaign Issue, Calls for a Mandate

The virus continues to loom over Mr. Trump as a major political liability. Fifty-seven percent of Americans said the president was doing a bad job dealing with the virus, and 52 percent said the United States’ response was worse than other countries’, according to a Monmouth University poll released Thursday.

And in Wisconsin, an important battleground state, 69 percent of voters said people should be required to wear masks in all public places, according to a poll released Tuesday by Marquette University Law School. Polling by Quinnipiac University last month found that four in five voters in Florida and Texas also supported mask mandates.

In addition to the briefing on the virus on Thursday, Mr. Biden and Ms. Harris also received a briefing on the economic situation, with experts appearing by videoconference in both sessions. Janet L. Yellen, the former Federal Reserve chair, was among the participants in the economic briefing, the Biden campaign said.

As for their own mask etiquette, Mr. Biden and Ms. Harris did not wear masks, but were seated at a distance, when reporters were briefly allowed into the meeting on the virus. When they later addressed reporters, they wore masks until they got to the lectern, and then put their masks back on after they finished speaking.

Ms. Harris tried to cast doubt on the Trump administration’s efforts to develop a vaccine in a speedy manner, suggesting that what mattered was when that vaccine would be available to the public.

“I think it’s important that the American people, looking at the election coming up, ask the current occupant of the White House: When am I going to get vaccinated?” Ms. Harris said. “Because there may be some grand gestures offered by the current president about a vaccine. But it really doesn’t matter until you can answer the question: When am I going to get vaccinated?”

As Mr. Biden and Ms. Harris put their focus on the pandemic, Mr. Trump continued to ridicule Ms. Harris in a television interview, auditioning one of his demeaning schoolyard nicknames — a go-to Trump tactic that some Republican officials worry will backfire among suburban women who will see such an attack as sexist.