President Trump and the physicians advising the federal pandemic response on Tuesday delivered a bleak outlook for the novel coronavirus’s spread across the country, predicting a best-case scenario of 100,000 to 240,000 fatalities in the United States and summoning all Americans to make additional sacrifices to slow the spread.

Trump adopted a newly somber and sedate tone — and contradicted many of his own previous assessments of the virus — as he instructed Americans to continue social distancing, school closures and other mitigation efforts for an additional 30 days and to think of the choices they make as matters of life and death.

Trump and his coronavirus task force members said that community mitigation practices in place for the past 15 days have worked and that extending them is essential. The mathematical modeling the White House presented suggests doing so could save hundreds of thousands of lives. Without community mitigation, the models predict, 1.5 million to 2.2 million Americans could die of covid-19, the disease the virus causes, though no time frames or other details were provided for the figures.

“Our country is in the midst of a great national trial unlike any it has ever faced before,” Trump said at an early evening news conference. He went on to call on every citizen to “make sacrifices” and every business to fulfill its “patriotic duty” to brace for even tougher days ahead.

“This is going to be a very painful — very, very painful two weeks,” Trump said. Sometime after April, he added, the country could transition back to normal with businesses reopening and people returning to work.

“It’s going to be like a burst of light, I really think, and I hope,” Trump said. “Our strength will be tested, our endurance will be tried, but America will answer with love and courage and ironclad resolve.”

Deborah Birx, a physician who is coordinating the White House coronavirus task force, delivered a slide show marking a stark difference in the spread of the virus in New York and New Jersey, where the number of cases has spiked, and in the other 48 states and the District.

Birx said the federal government’s goal over the next month is to control the outbreak in New York and New Jersey while staving off outbreaks in other states and metropolitan areas.

“If you had more New Yorks and New Jerseys — you know, Chicago, Detroit, L.A., Dallas, Houston, all of our major cities modeled like New York — that’s what gets us into trouble,” she said.

Birx noted the Detroit, Chicago and New Orleans areas, as well as the state of Massachusetts, as places with a troubling rise in cases. She said spikes there and in other cities can be prevented only with mitigation in every community coast to coast.

“There’s no magic bullet,” Birx said. “There’s no magic vaccine or therapy. It’s just behaviors — each of our behaviors translating into something that changes the course of this viral pandemic over the next 30 days.”

Anthony S. Fauci, another physician on the task force and director of the National Institute of Allergy and Infectious Diseases, said, “This is tough. People are suffering. People are dying. It’s inconvenient from a societal standpoint, from an economic standpoint, to go through this, but this is going to be the answer to our problems.”

Trump said the data — coupled with harrowing images of death in his hometown of New York — had an effect on him. Over the weekend, he heeded the advice of Birx and Fauci to extend the federal social distancing guidelines rather than rush to reopen the economy by Easter, April 12, as he had repeatedly suggested last week.

Although Trump’s decision effectively keeps much of the country shut down, he is only empowered to make recommendations. The authority to order closures or other restrictions lies primarily with state and local leaders.

Trump largely kept to a serious tone through much of Tuesday’s news conference, yet he veered off course at several moments. He riffed about the “total hoax” of his impeachment, dinged House Speaker Nancy Pelosi (D-Calif.) on homelessness in her San Francisco district, promised to never approve the Green New Deal and took jabs at New York Gov. Andrew M. Cuomo (D).

“For whatever reason, New York got off to a very late start, and we see what happens,” Trump said — an apparent rebuke of Cuomo’s leadership that overlooked the fact that the president himself had dismissed the threat of the virus.

In addition, Trump complained about Cuomo’s frequent and impassioned pleas for federal help in New York. “For some people, no matter how much you give, it’s never enough,” Trump said.

His daily news conferences have frequently been forums for self-congratulation, with the president repeatedly bragging about the work he and his administration have done responding to the pandemic, despite voluminous evidence that warning signs were ignored for weeks.

Monday’s performance — when Trump turned over his Rose Garden rostrum to an array of corporate executives, one by one, to praise him and to pitch their products — touched a nerve for one of his more prominent supporters. New York sports talk radio icon Mike Francesa, a longtime and vocal defender of Trump, delivered an on-air tirade Tuesday about the president’s leadership.

“Don’t give me the MyPillow guy doing a song-and-dance up here on a Monday afternoon when people are dying in Queens,” Francesa said. “Get the stuff made, get the stuff where it needs to go, and get the boots on the ground! Treat this like the crisis it is!”

Francesa seized upon Trump’s comments Monday that if the total number of coronavirus fatalities in the United States is between 100,000 and 200,000, that would count as a good job.

“How can you have a scoreboard that says 2,000 people have died and tell us, ‘It’s okay if another 198,000 die, that’s a good job,’ ” Francesa said. “How is that a good job in our country? It’s a good job if nobody else dies! Not if another 198,000 people die! So now 200,000 people are disposable?”

At Tuesday’s news conference, Trump painted an apocalyptic portrait of the country had people not been social distancing.

“You would’ve seen people dying on airplanes,” he said. “You would’ve seen people dying in hotel lobbies. You would’ve seen death all over.”

By comparison, the 1918 influenza epidemic killed an estimated 675,000 Americans at a time when the population was much lower. The 1957 influenza epidemic killed 70,000 to 116,000 people.

Trump said some of his business friends have advised him to “don’t do anything, just ride it out and think of it as the flu. But it’s not the flu. It’s vicious.”

Trump himself has drawn parallels between the coronavirus and the seasonal flu repeatedly, arguing as recently as last week that the flu is more deadly.

The data presented by the White House was based in part on publicly available models. One was created by the University of Washington’s Institute for Health Metrics and Evaluation, which projects the national strain on hospitals will peak on April 15. It also estimated deaths through this summer as 38,000 to 162,000 — a lower projection than some earlier models and the White House’s own estimate. But that model notably assumes every state enacts strict restrictions on residents; some states, such as Florida, have yet to do so. It also assumes the entire country will maintain these strict restrictions until summer, but Trump has extended the White House’s guidance only until April 30.

Another model cited by the White House was created by Imperial College in Britain and showed as many as 2.2 million U.S. deaths if absolutely no action was taken, 1.1 million if moderate mitigation strategies were adopted and an unspecified number if drastic measures were taken.

The actual death count in coming months will depend on a variety of factors that no one yet knows, including whether states can procure enough ventilators and other supplies, whether hospitals become overwhelmed, and whether people dutifully follow guidelines.

Birx said the models rely heavily on data from New York and New Jersey. Neither she nor other officials disclosed the underlying data and assumptions in the White House’s charts. One key question, for example, is what time period the projection of 100,000 to 240,000 deaths covers. If it is only the few months until summer, as is the case in at least one academic model, the true death toll will probably be larger.

Some of Trump’s allies made excuses Tuesday for why the president and his team were so slow to recognize the threat. The first U.S. coronavirus case was reported on Jan. 21, and reports of a deadly outbreak in China preceded that.

Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday that Trump’s impeachment distracted the administration’s attention from the emerging crisis, seeming to lay blame on the congressional Democrats who led the effort.

“It came up while we were, you know, tied down in the impeachment trial,” McConnell said in an interview with conservative talk-radio host Hugh Hewitt. “And I think it diverted the attention of the government because everything, every day was all about impeachment.”

Yet the timeline of Trump’s impeachment — he was acquitted by the Senate on Feb. 5 — does not align with the president’s nonchalance about the coronavirus, which continued for well more than a month after his acquittal.

McConnell’s suggestion that the president or his administration was distracted by impeachment also does not comport with Trump’s schedule. During that period, he held a number of “Keep America Great” campaign rallies and fundraisers across the country, as well as playing golf and socializing at his Mar-a-Lago Club in Florida.

Trump repeatedly dismissed the threat the virus posed. After news of the first U.S. case broke in January, Trump said, “We have it totally under control. . . . It’s going to be just fine.” Around the same time, Senate Minority Leader Charles E. Schumer (D-N.Y.) called on the federal government to declare the outbreak a public health emergency.

Trump continued to play down the danger of the coronavirus, saying in late February, “It’s going to disappear. One day, it’s like a miracle, it will disappear.”

Asked Tuesday about McConnell’s view that impeachment had distracted the president, Trump said he did not agree.

“I don’t think I would’ve done any better had I not been impeached,” Trump said. “I think that’s a great tribute to something. Maybe it’s a tribute to me.”

Yasmeen Abutaleb, Joel Achenbach, Colby Itkowitz, Ashley Parker and Felicia Sonmez contributed to this report.

White House projection of deaths due to covid-19

With mitigation: 100,000-240,000

Without mitigation: 1.5 million-2.2 million