While the politics have changed, the public health advice remains the same: Wear a mask in public.
Democratic presidential candidate Joe Biden wore a face mask at Memorial Day observances Monday, as the U.S. death toll from COVID-19, the disease caused by the coronavirus, neared 100,000.
President Donald Trump did not. Nor did he wear one during visits this month to a Ford Motor Company factory making ventilators, or a Honeywell factory making masks.
Biden called him an "absolute fool" for not doing so. "He's supposed to lead by example," he said.
Trump says he does not need to because he is tested every day and keeps his distance from others. Masks are required for the rest of the White House staff.
The difference between Trump and Biden highlights a wider political divide.
While three-quarters of Americans said they wear masks most or every time they leave the house, that figure falls to 58% for Republicans and 49% for Republican men, according to a recent Kaiser Family Foundation poll.
Unpublished research shows Republicans are also less likely to abide by social distancing recommendations.
"That makes me sad," said University of California, San Diego atmospheric chemistry professor Kim Prather. "There's no different laws of physics depending on what your party is."
Without a vaccine or treatment, face coverings are one of the few tools available to fight the COVID-19 pandemic.
Prather co-authored an article in the journal Science that says wearing masks should be universal, especially indoors.
Getting going
The partisan divide over masks will "get in the way of us getting our country back going," she said.
As the country reopens from lockdowns, experts have warned that infections and deaths are likely to increase again.
"I heard that one night on the news, and I thought, 'If we keep doing things the way that we're doing, then yeah,'" Prather said. "But I really believe that if we wear masks and are smart about it, we can go back out."
The risk of spreading the disease seems to be lower outside, where breezes blow particles away and sunlight probably knocks down the virus. Many restaurants are beginning to reopen with outdoor dining only.
There is still a risk, however, and masks would help, Prather said.
"If you are sitting next to someone outside that is infected, and they're talking to you for an hour, then you're going to inhale, potentially, a lot of this virus," she said.
The coronavirus spreads mainly through infected droplets. It also appears to spread through airborne particles, though it's not clear how much of a role this route plays.
Masks, even homemade ones, provide good protection from infected droplets. They can provide good protection from airborne particles if they fit tightly.
Protecting others
People should wear masks even if they don't feel sick, because they can spread the virus without showing symptoms themselves, Prather said.
"You wear a mask to protect others. That's how it is done," she said. The wearer gets some protection, but "it's more important that we limit our emissions just in case we're the sick one."
Masks need to fit correctly, and people need to know how to wear them, noted Josh Santarpia, associate professor of microbiology at the University of Nebraska Medical Center.
Santarpia said people with poorly fitting masks tend to touch their faces while trying to adjust them, which introduces more opportunities to get infected.
Usually, he said, some protection is better than none, but "if they're not doing it right, it could actually be worse than not doing anything at all."
There are some indications that masks may be helping as Americans start to move around more.
One of the leading disease forecasting groups, the University of Washington Institute for Health Metrics and Evaluation, said cases of COVID-19 have not increased as much as they would have expected as restrictions were lifted.
Though mask use is not part of their models at this point, it's a new area they are looking into.
"Using masks when in public appears to be another important tool in curbing COVID-19's toll," the group said.
