Biden Hits Trump’s Coronavirus Response: ‘He’s Worried About Looking Bad’

But there is little evidence so far that American voters are punishing Mr. Biden for his caution. A suite of polls from The New York Times and Siena College released this week show Mr. Biden leading Mr. Trump by 14 percentage points nationwide and with healthy leads in the battleground states of Arizona, Florida, Michigan, North Carolina, Pennsylvania and Wisconsin.

On Thursday Mr. Biden called on Americans to wear masks in public to protect themselves and others. He began his remarks wearing what has become his signature black face mask, which hung from his left ear for the opening minutes of his speech. As he finished and walked off stage, Mr. Biden reattached the mask.

“I know as Americans, it’s not something we’re used to, but it matters,” he said. “All the evidence from all over the world tells us it might be the single most effective thing we can do.”

Mr. Trump, who has refused to wear a mask in public and has appeared at political events at which most of his supporters have gone maskless as well, first said that he had ordered a slowdown in coronavirus testing during a speech he gave to a half-full arena Saturday in Tulsa, Okla. He said that more testing had led to more evidence of infection, which he said reflected poorly on the United States.

“So I said to my people, ‘Slow the testing down,’” Mr. Trump said.

Almost immediately, White House aides said the president had been joking. But later this week, Mr. Trump told reporters, “I don’t kid,” and reiterated his opposition to increased testing.

Mr. Biden called Mr. Trump’s response self-centered and compared him to a child who constantly complains about being unable to get what he wants.

“The president wants you to believe this is a choice between the economy and the public’s health,” Mr. Biden said. “He still hasn’t grasped the most basic fact of this crisis: To fix our economy, we have to get control of the virus.”