Chris Christie, the former governor of New Jersey who for several days this week helped the president prepare for the debate, said he has tested positive for the coronavirus.
Mr. Christie announced his condition on Saturday, becoming the latest of several Trump associates to say they had tested positive for the virus.
“I want to thank all of my friends and colleagues who have reached out to ask how I was feeling in the last day or two,” he said on Twitter.
In another tweet later on Saturday, Mr. Christie said he had checked himself in to the Morristown Medical Center in Morristown, N.J. on Saturday afternoon after consulting with his doctors.
“While I am feeling good and only have mild symptoms, due to my history of asthma we decided this is an important precautionary measure,” he said.
Mr. Christie’s statement came one day after Bill Stepien, President Trump’s campaign manager, tested positive for the virus.
Mr. Stepien, who was with Mr. Trump at the presidential debate in Cleveland on Tuesday, is experiencing mild symptoms and is in isolation, according to a person briefed on the matter.
Mr. Christie and Mr. Stepien were among several advisers who huddled with Mr. Trump and others for debate preparation from Sunday to Tuesday.
That group also included Hope Hicks, one of Mr. Trump’s closest advisers, and Kellyanne Conway, the former senior White House aide, both of whom have since tested positive.
No one wore masks during the preparation, Mr. Christie said.
Ms. Conway and Mr. Christie also attended a White House event on Sept. 26 announcing Mr. Trump’s nominee for the Supreme Court, Judge Amy Coney Barrett.
Others who attended and said they have tested positive include Senator Mike Lee, Republican of Utah; Senator Thom Tillis, Republican of North Carolina; and the Rev. John I. Jenkins, the president of the University of Notre Dame.