“I believed when I entered the White House grounds, that I had entered a safe zone, due to the testing that I and many others underwent every day,” Mr. Christie said in the statement. “I was wrong. I was wrong not to wear a mask at the Amy Coney Barrett announcement and I was wrong not to wear a mask at my multiple debate prep sessions with the president and the rest of the team.”

“I hope that my experience shows my fellow citizens that you should follow C.D.C. guidelines in public no matter where you are and wear a mask to protect yourself and others,” he said.

Unlike Mr. Trump, who has posited that he feels like “Superman” and left the hospital as quickly as possible, Mr. Christie said that the virus is “something to take very seriously. The ramifications are wildly random and potentially deadly.”