Privately, aides said, Mr. Trump knows things aren’t looking good for him — he just thinks the public polls are overstating the situation. His campaign does not conduct national polls, but aides have presented him with internal data about battleground states that show a closer race than the public polling numbers. His pollsters tell him regularly that he is in a close race and that there is more polling bias in the news media today than there was four years ago, a claim untethered to any measurable metric. They assure him that his base is still enthusiastically engaged and that the middle that might have been planning to vote for him in March has moved away through no fault of his own…

But aides said that even in private conversations, Mr. Trump has not let the reality of his current political standing fully sink in.

“No one’s ever come back from something like this,” said Lee Miringoff, director of the Marist College Institute for Public Opinion, referring to Mr. Biden’s polling lead over Mr. Trump. Indeed, it has been almost 25 years since Bill Clinton sustained such a gaping advantage over his opponent, Bob Dole, in 1996.