A U.S. mask mandate would probably face a legal challenge, but Biden, if elected, would have ways to promote it.

As the nation heads into what some public health experts warn could be a “dark winter” of coronavirus illness and death, a growing cadre is coalescing around Joseph R. Biden Jr.’s call for a “national mask mandate,” even as they concede such an effort would require much more than the stroke of a presidential pen.

Over the past week, a string of prominent public health experts — notably Dr. Anthony S. Fauci, the government’s top infectious disease specialist, and Dr. Scott Gottlieb, a former commissioner of food and drugs under President Trump — have said it is time to seriously consider a national mandate to curb the spread of the virus.

Overseas, President Vladimir V. Putin of Russia this week became the latest foreign leader to impose a national mandate for citizens to wear masks. Mr. Trump is opposed to a mandate, and Mr. Biden has conceded that a presidential order for all Americans to wear masks would almost certainly face — and likely fall to — a legal challenge.

Mr. Biden, who echoed warnings of a “dark winter” during the final presidential debate, is already using his bully pulpit to promote and reinforce a culture of mask wearing. If he is elected, he will almost certainly do more.

Mr. Biden has already said that, as president, he would mandate masks on all federal property, an executive order that could have wide reach. He could use his authority under federal transit law to require masks on public transportation. He could also prod governors who are resisting mask mandates to at least require masks in public buildings in their states.

But that is delicate political terrain in the United States, where Mr. Trump has turned the act of wearing a mask — or not wearing one — into a political statement. Public health and legal experts say it would be far better for Mr. Biden — or Mr. Trump, for that matter — to use his powers of persuasion to convince Americans that covering one’s face to protect against disease is a patriotic or civic-minded action.

“Instead of making it about the president’s coercive authority under law it should be about whether the president can support a norm that supports public health, which is in people’s self interest,” said Harold Koh, a law professor at Yale University and an expert in national security and human rights.

Experts say the scientific evidence is growing that face masks can considerably reduce the transmission of respiratory viruses like the one that causes Covid-19. Even when mask wearing does not prevent infection, it can reduce the severity of disease by diminishing the intensity of a person’s exposure to the virus. Research also shows that states that have passed mask mandates have had lower growth rates of Covid-19, beginning on the day the mandate was passed.

Even so, any hint of a sweeping federal requirement would “go over like a lead balloon,” and “divide and harden areas of the country in opposition,” said Joel White, a Republican strategist with expertise in health policy. Mr. White said the Trump administration’s policy, of letting state and local leaders decide about masks, is “a far better way to go.”

But that has not produced the kind of compliance that public health experts say is necessary to reduce the spread of the virus. As of last week, 33 states and the District of Columbia required mask-wearing in public, according to a list compiled by AARP. But in certain parts of the country, especially heavily Republican states, resistance is deep — even when cases are soaring.

— Sheryl Gay Stolberg The New York Times