President Biden tested positive for Covid on Wednesday, according to the White House, forcing him to cancel an event in Las Vegas and likely sidelining him for days following the conclusion of former President Donald J. Trump’s nominating convention on Thursday.

Karine Jean-Pierre, the White House press secretary, said in a statement that “earlier today, following his first event in Las Vegas, President Biden tested positive for COVID-19.” She added that “he is vaccinated and boosted and he is experiencing mild symptoms.”

She said Mr. Biden would travel from Las Vegas to Delaware, where he would self-isolate. “The White House will provide regular updates on the president’s status as he continues to carry out the full duties of the office while in isolation,” she added.
Mr. Biden leaned out of his limousine as he arrived at Air Force One to fly back to Delaware and gave reporters a thumbs-up. When they asked how he was feeling, he replied: “Good. I feel good.”

The illness — Mr. Biden’s third bout with Covid, including one rebound case — comes as some of his allies and supporters are continuing to press for him to drop out of the race, citing concerns about his ability to defeat Mr. Trump in November.

Having Covid will complicate the president’s ability to answer his critics, many of whom have said they want him to show that he still has the vigor and energy to prosecute the case against Mr. Trump in campaign rallies, interviews and other events.
If Mr. Biden continues to test positive for the next week or more, it could keep him off the campaign trail. And if he continues to have symptoms, it could hamper his ability to perform well in interviews that he could do from home.

Mr. Biden’s doctor, Kevin O’Connor, said in a release from the White House that the president had a runny nose, a cough and “general malaise” starting Wednesday afternoon. Dr. O’Connor said that he was given a rapid test for Covid because he was still not feeling well later in the day, and the results were positive for the virus.
“Given this, the president will be self-isolating in accordance with C.D.C. guidance for symptomatic individuals,” he wrote. He added that the president’s temperature was normal and his blood oxygen was normal as well.

Mr. Biden has received an initial dose of Paxlovid, a treatment that may lessen the symptoms of the virus.
Before announcing the test results, Mr. Biden stopped at a restaurant earlier in the day. Reporters with him said he looked slower and a bit more pale than the day before, when he delivered a speech at the N.A.A.C.P. convention.

Reporters traveling with the president said that there was a lengthy, unexplained delay in the afternoon leading up to the president’s scheduled appearance at the annual conference for UnidosUS, a Latino civil rights and advocacy organization.
Reporters repeatedly asked the president’s press aides for the reason of the delay.

The first indication of Mr. Biden’s diagnosis came not from the White House but from Janet Murguía, the chief executive of Unidos, who addressed the conference more than an hour after Mr. Biden had been expected to speak.
“Regrettably, I was just on the phone with President Biden. And he shared his deep disappointment at not being able to join us this afternoon,” she said. “The president has been at many events, as we all know, and he just tested positive for Covid.”

Mr. Biden first tested positive for Covid in July of 2022 and experienced mild symptoms for about a week. After testing negative, Mr. Biden started testing positive about a week later in what doctors said was a rebound case of Covid.