Trump wears a mask on a visit to a military medical center.

President Trump on Saturday wore a mask in public for the first time, after repeated urging from aides that it was a necessary message to send to Americans resistant to covering their faces.

Mr. Trump wore a dark mask affixed with the presidential seal during a visit to the Walter Reed National Military Medical Center, where he planned to visit wounded troops. He was surrounded by Secret Service agents and others also wearing masks.

Anticipation over whether he would wear a mask had been building, after the president had repeatedly dismissed suggestions that he wear a mask, frequently appearing in public spaces without one, mocking those who did and ignoring public health rules in several states.

But as the pandemic has spread to states with large numbers of Republican voters, Mr. Trump had signaled more recently that he was more open to masks, and told reporters he planned to wear a mask on the visit to the medical center. Before going there, he stopped to speak with reporters at the White House.

“I’ll probably have a mask, if you must know,” Mr. Trump said. “I think when you’re in a hospital, especially in that particular setting where you’re talking to a lot of soldiers and people that in some cases just got off the operating tables. I think it’s a great thing to wear a mask. I’ve never been against masks, but I do believe they have a time and a place.”

Weeks ago, the president took a mask off before seeing reporters during a tour of a Ford plant in Michigan, in violation of factory policy, which he said he did to avoid being photographed with it on. Last month he refused to wear a mask at a factory in Maine that produced coronavirus testing swabs, forcing the manufacturer to dump products made during a demonstration for the president. And in May, he did not wear a mask during a visit to a Honeywell mask factory in Phoenix, Ariz., despite signage in the building asking visitors and workers to do so at all times.

Masks were scarce at recent Trump campaign events in Oklahoma, Arizona and South Dakota.

In contrast to Mr. Trump’s reluctance, a growing number of governors, both Republican and Democratic, and even Vice President Mike Pence have taken up the cause in recent weeks. Gov. Doug Ducey of Arizona and Gov. Brian Kemp of Georgia are among the chorus of Republican officials who have encouraged their constituents to actively cover their faces in public.

In Louisiana, Gov. John Bel Edwards on Saturday said almost all residents must wear a mask in public. Gov. Greg Abbott of Texas, a Republican, recently issued a similar order.

Senators Lisa Murkowski of Alaska, Lamar Alexander of Tennessee and Mitt Romney of Utah have also called on the president to wear face coverings, at least as a symbolic gesture. Gov. Andrew Cuomo has also implored the president to wear one.

In an interview this month, Mr. Trump said he “would wear one if I were in a group of people and I was close,” adding that he “sort of liked” the way he looked.

“It was a dark black mask,” he said at the time, “and I thought it looked OK. I looked like the Lone Ranger.”