“I want every American to be prepared for the hard days that lie ahead,” said Mr. Trump, who answered questions for more than two hours and predicted that there would be “light at the end of the tunnel,” but warned that “we’re going to go through a very tough two weeks.”

Dr. Fauci and Dr. Birx showed charts indicating that coronavirus cases in New York and New Jersey had risen far higher than in other parts of the country, a fact that they said gave them hope that the overall number of deaths might be lower if people in the rest of the states followed the guidelines for at least the next month.

Mr. Trump displayed none of the carefree dismissiveness that characterized his reaction to the virus in February and early March, when he repeatedly said that “we have it totally under control” and that “it’s going to be just fine.” For most of the briefing on Tuesday, the president appeared to understand the severity of the potentially grave threat to the country.

But asked about whether the impeachment effort had distracted him in the early days of the pandemic, Mr. Trump reverted to form, lashing out at Democrats and once again calling it “a phony impeachment” and “a hoax.” He acknowledged that he might have been distracted, but insisted that he still deserved an “A+” for his effort to combat the virus.

But outbreaks in New Orleans, Detroit and other cities are growing quickly, and experts say it is unclear whether social distancing measures can stop them from rising even more in the next few weeks. Recent estimates in Florida suggest that it may be entering a phase of exponential growth.