The president, who had been warned aboard Air Force One that the crowds at the arena were smaller than expected, was stunned, and he yelled at aides backstage while looking at the endless rows of empty blue seats in the upper bowl of the stadium, according to four people familiar with what took place. Brad Parscale, the campaign manager who had put the event together, was not present…

Exactly what went wrong was still being dissected on Sunday. But a broad group of advisers and associates acknowledged to one another that Mr. Trump had not been able to will public opinion away from fears about the spread of the coronavirus in an indoor space. And they conceded that myriad polls showing Mr. Trump’s eroded standing were not fake, and that he might be on course to lose to former Vice President Joseph R. Biden Jr., the presumptive Democratic nominee, in November…

Campaign officials on Sunday privately admitted that many people who had signed up to attend the event were not supporters but online tricksters. One campaign adviser claimed that “troll data” was still useful, claiming it would help the campaign avoid the same pitfall in the future.

The adviser said that the data could be put into the system to “tighten up the formula used to determine projected attendance for rallies.”