Some conservatives have said they did not believe that Americans would ever accept wide usage. Michael Dougherty, a conservative writer at National Review, wrote that Americans would “quickly feel that masks are ridiculous, menacing, or an imposition on life, then conclude they must be temporary.”

Mr. Trump’s personal hesitance also underscored questions about whether other politicians or media personalities would choose to wear masks while appearing in public.

Outside of the White House, the move toward masks accelerated quickly this week. On Friday, after Gov. Tom Wolf of Pennsylvania, a Democrat, urged residents of his state to wear masks if they ventured out of their homes, the state’s health secretary reiterated that staying at home — away from groups of people — remained the most effective way to ensure that the virus would not spread.

“A mask isn’t a pass to go back to work, or go visit friends, or go socialize,” said the health secretary, Dr. Rachel Levine.

At the World Health Organization briefing on Friday, Dr. Michael J. Ryan, the executive director of the health emergency program, said that while the W.H.O. still recommended masks only for front-line health workers and those who are sick or caring for someone who is, “we can certainly see circumstances in which the use of masks, both homemade or cloth masks, at community level may help in an overall comprehensive response to this disease.”

Senator Patrick J. Toomey, Republican of Pennsylvania, has been one of the most vocal supporters of wearing masks. In an interview from the basement of his home, where he is isolating because of his recent contact with lawmakers who tested positive for the virus, he said wearing masks would help limit the effect of it.

“It just makes sense to have some kind of physical barrier that would reduce the droplets that are released when people speak and breathe,” Mr. Toomey said. “The idea is to protect everyone else. My mask protects you. Your mask protects me.”