Mr. Trump asserted that had he not blocked most travelers from China, the United States would have reached closer to the maximum projected death toll of up to 2.2 million. “When you look at it could have been 2.2 million people died and more if we did nothing, if we just did nothing,” he said, then he and the country “have done a great job.” In effect, he seemed to be setting up the argument that any death toll below that will be a validation of his handling of the crisis.

Whatever the eventual number will be, the pandemic of 2020 seems likely to rank with the deadliest of the past century. The worst came in 1918-20 and killed about 675,000 Americans, accounting for many of the military deaths attributed to World War I. Another pandemic in 1957-58 killed about 116,000 in the United States, and one in 1968 killed about 100,000. The H1N1 virus in 2009, for which Mr. Trump has assailed Mr. Obama’s response, killed only 12,000. The ordinary flu has resulted in 12,000 to 61,000 deaths annually since 2010.

Mr. Trump and his administration have stepped up efforts in recent weeks, expanding testing and seeking to work with governors to address shortages of ventilators, masks and other medical equipment. The president has dispatched medical ships and Army engineers to help, moved to force General Motors to manufacture more ventilators and, after flirting with an early reopening, extended social distancing guidelines until the end of April. Governors have both welcomed the help and complained that it is still inadequate.

For much of Tuesday’s marathon two-hour and 11-minute briefing, the longest single public appearance of his presidency, according to Factba.se, which monitors his activities, Mr. Trump took on a more somber approach as the scale of the fatalities seemed to sink in.

He jousted to some degree with Mr. Acosta and Yamiche Alcindor of “PBS NewsHour,” two of his favorite foils, but he was more restrained with them than usual and avoided some of the more incendiary language he often uses.

Yet he could not resist forever. By the time the briefing ended, he had lapsed back into complaints about the impeachment “hoax” and renewed his attacks on critics like James B. Comey, the former F.B.I. director, and Mr. Comey’s onetime deputy, Andrew G. McCabe, while congratulating himself for his performance. “Did it divert my attention?” the president asked of the impeachment. “I think I’m getting A-pluses for the way I handled myself during a phony impeachment.”

Still, Mr. Trump, rarely a reflective person in public, mused about the human toll of the pandemic more than he did in the early weeks of the crisis apparently because it has hit his own circle. As he has in the past couple of days, he referred to an overwhelmed hospital in his childhood home of Queens and an unidentified friend he said had been hospitalized with the virus.