Ms. Hicks, a senior aide and the closest known person to the president to have contracted the virus, traveled with him this week aboard Air Force One.
President Trump said late Thursday on Twitter that he and the first lady, Melania Trump, would begin the “quarantine process” as they await results of coronavirus tests they took after a senior adviser who is often with the couple tested positive.
It was not immediately clear what Mr. Trump meant by the “quarantine process,” and White House officials declined to immediately provide any guidance. As of 11:30 p.m., the president still had a full day of events planned for Friday, including a private fund-raiser at the Trump International Hotel in Washington and a campaign rally in Florida. He was also scheduled to hold a call with “vulnerable seniors” about the coronavirus.
And even if Mr. Trump first tests negative for the virus, the incubation period for it can last nearly two weeks, meaning he will not be certain for several days that he does not have it.
The frantic moves by the president and his aides came after Bloomberg News reported that the aide, Hope Hicks, one of Mr. Trump’s most senior advisers and someone who traveled with him several times this week, had tested positive. Two people briefed on her condition said she had symptoms.
The White House typically uses a rapid coronavirus test that produces results within a short period of time. Two hours before that tweet, Mr. Trump told the Fox News host Sean Hannity that he had been tested and was awaiting results, leaving the idea that he might enter into some kind of quarantine as an open question.
Ms. Hicks is the closest known person to the president to have contracted the virus, which causes Covid-19. That an adviser with such proximity to other aides has the virus raised alarms inside the White House, where officials worried that other top officials may have been infected.
Officials at the White House have known about Ms. Hicks’s likely diagnosis since Wednesday evening, when she traveled with the president aboard Air Force One to Minnesota for a campaign rally in Duluth. One person familiar with the events on Wednesday said that Ms. Hicks began exhibiting minor symptoms around the time of Mr. Trump’s rally, and that she was quarantined on the return flight to Washington and then disembarked from the back entrance of the plane.
The positive diagnosis came on Thursday, a person familiar with her case said.
Ms. Hicks also accompanied Mr. Trump to the first presidential debate against former Vice President Joseph R. Biden Jr., the Democratic nominee, on Tuesday in Ohio. Attendance at the event was significantly scaled back to prevent the spread of the coronavirus; people familiar with the planning said that Mr. Trump had wanted people in the audience.
Speaking to Mr. Hannity, Mr. Trump appeared to blame soldiers and law enforcement officials who he said wanted to give her hugs and thank her for her work at the White House.
“She’s a very warm person,” the president said. “When soldiers and law enforcement come up to her,” he said, she does not want to reject their entreaties.
“I was surprised to hear with Hope,” the president said. He added that he and Mrs. Trump were both tested on Thursday for the coronavirus.
A White House spokesman, Judd Deere, would not comment directly on Ms. Hicks’s condition.
“The president takes the health and safety of himself and everyone who works in support of him and the American people very seriously,” Mr. Deere said. The White House operations team worked closely with the president’s doctor, among others, he added, “to ensure all plans and procedures incorporate current C.D.C. guidance and best practices for limiting Covid-19 exposure to the greatest extent possible.”
There is no one closer to Mr. Trump than Ms. Hicks, who returned to the White House this year after leaving her position as communications director in 2018. She has overseen significant aspects of the president’s daily schedule, and she works closely with Mr. Trump’s son-in-law and senior adviser, Jared Kushner.
Ms. Hicks was one of the few White House senior aides seen sporadically wearing masks in meetings, according to her colleagues. But she also has been photographed around Mr. Trump without one.
Mr. Trump has faced fierce criticism from health experts and his Democratic opponents for his handling of the coronavirus crisis over the past nine months. The virus has claimed more than 200,000 lives in the United States and has infected more than seven million people, forcing businesses, schools and much of daily life to shut down.
The president repeatedly played down the pandemic in its early days, telling the public that the virus would disappear “like magic” and mocking the recommendation by the Centers for Disease Control and Prevention to wear masks. More recently, Mr. Trump has disregarded advice from his own health experts and has pressured them to produce a vaccine, saying publicly that it would come before Election Day.
He has also frequently mocked Mr. Biden for wearing a mask, despite clear guidance from his own government that they offer significant protection.
Mr. Trump was playing down the virus even as he was aware that Ms. Hicks had fallen ill. In a virtual appearance at the annual Alfred E. Smith Memorial Foundation Dinner held in New York on Thursday night, Mr. Trump falsely claimed, “I just want to say that the end of the pandemic is in sight.”
Polls show that Mr. Trump’s handling of the pandemic has become a leading issue for many voters as the president looks to win another four years in the White House.
The president has frequently waved aside concerns about contracting the coronavirus, telling reporters — who uniformly wear masks while at the White House — that everyone around him is tested before they are allowed to be with him.
Reporters who are scheduled to travel with the president are required to get coronavirus tests before they are permitted on Air Force One or into Oval Office meetings. Other visitors are also screened before they are allowed near him.
Two people briefed about Ms. Hicks’s positive diagnosis said that several White House officials would be required to quarantine because they had come into contact with her. But at least one person who had been around Ms. Hicks, the White House press secretary, Kayleigh McEnany, held a news conference at the White House on Thursday morning.
However, as the president traveled in the afternoon for a fund-raiser at his private club in Bedminster, N.J., he had fewer aides than normal accompanying him, aides said. And some who had been around Ms. Hicks this week were seen wearing masks.
In May, the president revealed that he had taken a two-week course of an antimalarial drug called hydroxychloroquine, which he had frequently promoted as a way to prevent or cure Covid-19. At the time, his doctor said he had not tested positive and had chosen to take the drug as a preventive measure.
Later studies showed that the drug lacked effectiveness. In July, the Food and Drug Administration cautioned against its use. A study announced on Wednesday showed that the drug did not prevent infections among health care workers.
The president and his advisers inside the West Wing have largely shrugged off concerns about contracting the virus, even though three people, including two of his top aides, already have.
Katie Miller, the communications director for Vice President Mike Pence, tested positive and became ill in May, later revealing that she is pregnant. She returned to the White House at the end of that month. Mr. Trump’s personal military valet also tested positive in May.
Robert C. O’Brien, the president’s national security adviser, tested positive in July. He said he had mild symptoms and self-isolated for a period of time. None of those cases prompted meaningful changes of protocols for how the president and his staff interact.
But despite those cases, Mr. Trump and many of those around him do not regularly wear masks. Mark Meadows, the White House chief of staff, rarely uses one and was photographed interacting with people on a rope line without one.
Mr. Meadows and Mr. Trump say they feel protected because they are tested for the virus daily.
The president’s campaign has also taken few precautions regarding the virus, regularly holding rallies where the president’s supporters, journalists, White House staff, security and others are kept close together for hours. Many of those attending the events, including the president’s staff, do not wear masks.
Most of the rallies have been either outdoors or in airport hangars open to the outdoors. But at least one, in Tulsa, Okla., was held indoors and became what health officials there said was the likely source of a surge in the virus several weeks later.
In addition, eight members of Mr. Trump’s campaign staff who worked at the Tulsa rally tested positive for the coronavirus afterward, raising concerns about an outbreak among people close to the president.